Re: Possible memory allocation deadlock in kmem_alloc and hung task in xfs_log_commit_cil and xlog_cil_push

[Gavin Guo]

On Sun, Aug 30, 2015 at 6:31 AM, Dave Chinner <david@xxxxxxxxxxxxx> wrote:
> On Fri, Aug 28, 2015 at 08:54:04PM +0800, Gavin Guo wrote:
>> On Wed, Jul 8, 2015 at 7:37 AM, Dave Chinner <david@xxxxxxxxxxxxx> wrote:
>> > On Tue, Jul 07, 2015 at 05:29:43PM +0800, Gavin Guo wrote:
>> >> Hi all,
>> >>
>> >> Recently, we observed that there is the error message in
>> >> Ubuntu-3.13.0-48.80:
>> >>
>> >> "XFS: possible memory allocation deadlock in kmem_alloc (mode:0x8250)"
>> >>
>> >> repeatedly shows in the dmesg. Temporarily, our workaround is to tune the
>> >> parameters, such as, vfs_cache_pressure, min_free_kbytes, and dirty_ratio.
>> >>
>> >> And we also found that there are different error messages regarding the
>> >> hung tasks which happened in xfs_log_commit_cil and xlog_cil_push.
>> >>
>> >> The log is available at: http://paste.ubuntu.com/11835007/
>> >>
>> >> The following link seems the same problem we suffered:
>> >>
>> >> XFS hangs with XFS: possible memory allocation deadlock in kmem_alloc
>> >> http://oss.sgi.com/archives/xfs/2015-03/msg00172.html
>> >>
>> >> I read the mail and found that there might be some modification regarding
>> >> to move the memory allocation outside the ctx lock. And I also read the
>> >> latest patch from February of 2015 to see if there is any new change
>> >> about that. Unfortunately, I didn't find anything regarding the change (may
>> >> be I'm not familiar with the XFS, so didn't find the commit). If it's
>> >> possible for someone who is familiar with the code to point out the commits
>> >> related to the bug if already exist or any status about the plan.
>> >
>> > No commits - the approach I thought we might be able to take to
>> > avoid the problem didn't work out. I have another idea of how we
>> > might solve the problem, but I haven't ad a chance to prototype it
>> > yet.
>>
>> I have read the code for a while and still can't figure out how to fix.
>> My current understanding is that the problem is Buddy system is running out
>> of memory so the XFS kmem_alloc(),
>>
>>   called by xfs_log_commit_cil->
>>                 xlog_cil_insert_items->
>>                 xlog_cil_insert_format_items->
>>                 kmem_zalloc,
>>
>> fail and stuck in the while loop and retry. There are also 2 other threads
>> running in the same time:
>>
>> 1). xfs_log_commit_cil->down_read(&cil->xc_ctx_lock);
>>
>> 2). xlog_cil_push->down_write(&cil->xc_ctx_lock);
>>
>> So, the both threads are blocked and waiting for the first kmem_zalloc() to
>> succeed.
>>
>> However, if there is a way to decrease the memory request or if it's
>> possible to elaborate more on the idea you mentioned. I know it's a
>> problem which cannot be solved in a short time. And I'd like to help if
>> there is any possibility.
>
> This is the patch I'm currently working on. It doesn't work
> completely yet, but it will give you an idea of how the problem
> needs tobe solved (read the big comment in the patch).
>
> -Dave.
> --
> Dave Chinner
> david@xxxxxxxxxxxxx
>
> ---
>  fs/xfs/xfs_buf_item.c     |   1 +
>  fs/xfs/xfs_dquot.c        |   1 +
>  fs/xfs/xfs_dquot_item.c   |   2 +
>  fs/xfs/xfs_extfree_item.c |   2 +
>  fs/xfs/xfs_inode_item.c   |   1 +
>  fs/xfs/xfs_log_cil.c      | 228 ++++++++++++++++++++++++++++++++++------------
>  6 files changed, 177 insertions(+), 58 deletions(-)
>
> diff --git a/fs/xfs/xfs_buf_item.c b/fs/xfs/xfs_buf_item.c
> index 1816334..64cd236 100644
> --- a/fs/xfs/xfs_buf_item.c
> +++ b/fs/xfs/xfs_buf_item.c
> @@ -831,6 +831,7 @@ xfs_buf_item_free(
>         xfs_buf_log_item_t      *bip)
>  {
>         xfs_buf_item_free_format(bip);
> +       kmem_free(bip->bli_item->li_lv_shadow);
>         kmem_zone_free(xfs_buf_item_zone, bip);
>  }
>
> diff --git a/fs/xfs/xfs_dquot.c b/fs/xfs/xfs_dquot.c
> index 4143dc7..b9e7dda 100644
> --- a/fs/xfs/xfs_dquot.c
> +++ b/fs/xfs/xfs_dquot.c
> @@ -74,6 +74,7 @@ xfs_qm_dqdestroy(
>  {
>         ASSERT(list_empty(&dqp->q_lru));
>
> +       kmem_free(dqp->qli_item.li_lv_shadow);
>         mutex_destroy(&dqp->q_qlock);
>         kmem_zone_free(xfs_qm_dqzone, dqp);
>
> diff --git a/fs/xfs/xfs_dquot_item.c b/fs/xfs/xfs_dquot_item.c
> index 814cff9..2c7a162 100644
> --- a/fs/xfs/xfs_dquot_item.c
> +++ b/fs/xfs/xfs_dquot_item.c
> @@ -370,6 +370,8 @@ xfs_qm_qoffend_logitem_committed(
>         spin_lock(&ailp->xa_lock);
>         xfs_trans_ail_delete(ailp, &qfs->qql_item, SHUTDOWN_LOG_IO_ERROR);
>
> +       kmem_free(qfs->qql_item.li_lv_shadow);
> +       kmem_free(lip->li_lv_shadow);
>         kmem_free(qfs);
>         kmem_free(qfe);
>         return (xfs_lsn_t)-1;
> diff --git a/fs/xfs/xfs_extfree_item.c b/fs/xfs/xfs_extfree_item.c
> index adc8f8f..3842418 100644
> --- a/fs/xfs/xfs_extfree_item.c
> +++ b/fs/xfs/xfs_extfree_item.c
> @@ -40,6 +40,7 @@ void
>  xfs_efi_item_free(
>         struct xfs_efi_log_item *efip)
>  {
> +       kmem_free(efip->efi_item.li_lv_shadow);
>         if (efip->efi_format.efi_nextents > XFS_EFI_MAX_FAST_EXTENTS)
>                 kmem_free(efip);
>         else
> @@ -329,6 +330,7 @@ static inline struct xfs_efd_log_item *EFD_ITEM(struct xfs_log_item *lip)
>  STATIC void
>  xfs_efd_item_free(struct xfs_efd_log_item *efdp)
>  {
> +       kmem_free(efdp->efd_item.li_lv_shadow);
>         if (efdp->efd_format.efd_nextents > XFS_EFD_MAX_FAST_EXTENTS)
>                 kmem_free(efdp);
>         else
> diff --git a/fs/xfs/xfs_inode_item.c b/fs/xfs/xfs_inode_item.c
> index bf13a5a..39ca237 100644
> --- a/fs/xfs/xfs_inode_item.c
> +++ b/fs/xfs/xfs_inode_item.c
> @@ -577,6 +577,7 @@ void
>  xfs_inode_item_destroy(
>         xfs_inode_t     *ip)
>  {
> +       kmem_free(ip->ili_item->li_lv_shadow);
>         kmem_zone_free(xfs_ili_zone, ip->i_itemp);
>  }
>
> diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
> index abc2ccb..ab4b98c 100644
> --- a/fs/xfs/xfs_log_cil.c
> +++ b/fs/xfs/xfs_log_cil.c
> @@ -79,6 +79,145 @@ xlog_cil_init_post_recovery(
>         log->l_cilp->xc_ctx->sequence = 1;
>  }
>
> +static inline int
> +xlog_cil_iovec_space(
> +       uint    niovecs)
> +{
> +       return round_up((sizeof(struct xfs_log_vec) +
> +                                       niovecs * sizeof(struct xfs_log_iovec)),
> +                       sizeof(uint64_t));
> +}
> +
> +/*
> + * Allocate or pin log vector buffers for CIL insertion.
> + *
> + * The CIL currently uses disposable buffers for copying a snapshot of the
> + * modified items into the log during a push. The biggest problem with this is
> + * the requirement to allocate the disposable buffer during the commit if:
> + *     a) does not exist; or
> + *     b) it is too small
> + *
> + * If we do this allocation within xlog_cil_insert_format_items(), it is done
> + * under the xc_ctx_lock, which means that a CIL push cannot occur during
> + * the memory allocation. This means that we have a potential deadlock situation
> + * under low memory conditions when we have lots of dirty metadata pinned in
> + * the CIL and we need a CIL commit to occur to free memory.
> + *
> + * To avoid this, we need to move the memory allocation outside the
> + * xc_ctx_lock(), but because the log vector buffers are disposable, that opens
> + * up a TOCTOU race condition w.r.t. the CIL commiting and removing the log
> + * vector buffers between the check and the formatting of the item into the
> + * log vector buffer within the xc_ctx_lock.
> + *
> + * Because the log vector buffer needs to be unchanged during the CIL push
> + * process, we cannot share the buffer between the transaction commit (which
> + * modifies the buffer) and the CIL push context that is writing the changes
> + * into the log. This means skipping preallocation of buffer space is
> + * unreliable, but we most definitely do not want to be allocating and freeing
> + * buffers unnecessarily during commits when overwrites can be done safely.
> + *
> + * The simplest solution to this problem is to allocate a shadow buffer when a
> + * log item is committed for the second time, and then to only use this buffer
> + * if necessary. The buffer can remain attached to the log item until such time
> + * it is needed, and this is the buffer that is reallocated to match the size of
> + * the incoming modification. Then during the formatting of the item we can swap
> + * the active buffer with the new one if we can't reuse the existing buffer. We
> + * don't free the old buffer as it may be reused on the next modification if
> + * it's size is right, otherwise we'll free and reallocate it at that point.
> + *
> + * This function builds a vector for the changes in each log item in the
> + * transaction. It then works out the length of the buffer needed for each log
> + * item, allocates them and attaches the vector to the log item in preparation
> + * for the formatting step which occurs under the xc_ctx_lock.
> + *
> + * While this means the memory footprint goes up, it avoids the repeated
> + * alloc/free pattern that repeated modifications of an item would otherwise
> + * cause, and hence minimises the CPU overhead of such behaviour.
> + */
> +static void
> +xfs_cil_item_alloc_shadow_lvbufs(
> +       struct xlog             *log,
> +       struct xfs_trans        *tp)
> +{
> +       list_for_each_entry(lidp, &tp->t_items, lid_trans) {
> +               struct xfs_log_item *lip = lidp->lid_item;
> +               struct xfs_log_vec *lv;
> +               struct xfs_log_vec *old_lv;
> +               int     niovecs = 0;
> +               int     nbytes = 0;
> +               int     buf_size;
> +               bool    ordered = false;
> +
> +               /* Skip items which aren't dirty in this transaction. */
> +               if (!(lidp->lid_flags & XFS_LID_DIRTY))
> +                       continue;
> +
> +               /* get number of vecs and size of data to be stored */
> +               lip->li_ops->iop_size(lip, &niovecs, &nbytes);
> +
> +               /*
> +                * Ordered items need to be tracked but we do not wish to write
> +                * them. We need a logvec to track the object, but we do not
> +                * need an iovec or buffer to be allocated for copying data.
> +                */
> +               if (niovecs == XFS_LOG_VEC_ORDERED) {
> +                       ordered = true;
> +                       niovecs = 0;
> +                       nbytes = 0;
> +               }
> +
> +               /*
> +                * We 64-bit align the length of each iovec so that the start
> +                * of the next one is naturally aligned.  We'll need to
> +                * account for that slack space here. Then round nbytes up
> +                * to 64-bit alignment so that the initial buffer alignment is
> +                * easy to calculate and verify.
> +                */
> +               nbytes += niovecs * sizeof(uint64_t);
> +               nbytes = round_up(nbytes, sizeof(uint64_t));
> +
> +               /* grab the old item if it exists for reservation accounting */
> +               old_lv = lip->li_lv;
> +
> +               /*
> +                * The data buffer needs to start 64-bit aligned, so round up
> +                * that space to ensure we can align it appropriately and not
> +                * overrun the buffer.
> +                */
> +               buf_size = nbytes + xlog_cil_iovec_space(niovecs);
> +
> +               /*
> +                * if we have no shadow buffer, or it is too small, we need to
> +                * reallocate it.
> +                */
> +               if (!lip->li_lv_shadow ||
> +                   buf_size <= lip->li_lv_shadow->lv_size) {
> +
> +                       kmem_free(lip->li_lv_shadow);
> +
> +                       lv = kmem_zalloc(buf_size, KM_SLEEP|KM_NOFS);
> +                       lv->lv_item = lip;
> +                       lv->lv_size = buf_size;
> +                       if (ordered)
> +                               lv->lv_buf_len = XFS_LOG_VEC_ORDERED;
> +                       else
> +                               lv->lv_iovecp = (struct xfs_log_iovec *)&lv[1];
> +                       lip->li_lv_shadow = lv;
> +               } else {
> +                       /* same or smaller, optimise common overwrite case */
> +                       lv = lip->li_lv_shadow;
> +               }
> +
> +               /* Ensure the lv is set up according to ->iop_size */
> +               lv->lv_niovecs = niovecs;
> +
> +               /* The allocated data region lies beyond the iovec region */
> +               lv->lv_buf_len = 0;
> +               lv->lv_bytes = 0;
> +               lv->lv_buf = (char *)lv + xlog_cil_iovec_space(niovecs);
> +       }
> +
> +}
>  /*
>   * Prepare the log item for insertion into the CIL. Calculate the difference in
>   * log space and vectors it will consume, and if it is a new item pin it as
> @@ -101,7 +240,8 @@ xfs_cil_prepare_item(
>         /*
>          * If there is no old LV, this is the first time we've seen the item in
>          * this CIL context and so we need to pin it. If we are replacing the
> -        * old_lv, then remove the space it accounts for and free it.
> +        * old_lv, then remove the space it accounts for and make it the shadow
> +        * buffer for later freeing.
>          */
>         if (!old_lv)
>                 lv->lv_item->li_ops->iop_pin(lv->lv_item);
> @@ -110,7 +250,7 @@ xfs_cil_prepare_item(
>
>                 *diff_len -= old_lv->lv_bytes;
>                 *diff_iovecs -= old_lv->lv_niovecs;
> -               kmem_free(old_lv);
> +               lip->li_lv_shadow = old_lv;
>         }
>
>         /* attach new log vector to log item */
> @@ -134,11 +274,13 @@ xfs_cil_prepare_item(
>   * write it out asynchronously without needing to relock the object that was
>   * modified at the time it gets written into the iclog.
>   *
> - * This function builds a vector for the changes in each log item in the
> - * transaction. It then works out the length of the buffer needed for each log
> - * item, allocates them and formats the vector for the item into the buffer.
> - * The buffer is then attached to the log item are then inserted into the
> - * Committed Item List for tracking until the next checkpoint is written out.
> + * This function takes the prepared log vectors attached to each log item, and
> + * formats the changes into the log vector buffer. The buffer it uses is
> + * dependent on the current state of the vector in the CIL - the shadow lv is
> + * guaranteed to be large enough for the current modification, but we will only
> + * use that if we can't reuse the existing lv. If we can't reuse the existing
> + * lv, then simple swap it out for the shadow lv. We don't free it - that is
> + * done lazily either by th enext modification or the freeing of the log item.
>   *
>   * We don't set up region headers during this process; we simply copy the
>   * regions into the flat buffer. We can do this because we still have to do a
> @@ -171,7 +313,8 @@ xlog_cil_insert_format_items(
>         list_for_each_entry(lidp, &tp->t_items, lid_trans) {
>                 struct xfs_log_item *lip = lidp->lid_item;
>                 struct xfs_log_vec *lv;
> -               struct xfs_log_vec *old_lv;
> +               struct xfs_log_vec *old_lv = NULL;
> +               struct xfs_log_vec *shadow;
>                 int     niovecs = 0;
>                 int     nbytes = 0;
>                 int     buf_size;
> @@ -181,49 +324,19 @@ xlog_cil_insert_format_items(
>                 if (!(lidp->lid_flags & XFS_LID_DIRTY))
>                         continue;
>
> -               /* get number of vecs and size of data to be stored */
> -               lip->li_ops->iop_size(lip, &niovecs, &nbytes);
> -
> -               /* Skip items that do not have any vectors for writing */
> -               if (!niovecs)
> -                       continue;
> -
>                 /*
> -                * Ordered items need to be tracked but we do not wish to write
> -                * them. We need a logvec to track the object, but we do not
> -                * need an iovec or buffer to be allocated for copying data.
> +                * The formatting size information is already attached to
> +                * the shadow lv on the log item.
>                  */
> -               if (niovecs == XFS_LOG_VEC_ORDERED) {
> +               if (shadow->lv_buf_len == XFS_LOG_VEC_ORDERED)
>                         ordered = true;
> -                       niovecs = 0;
> -                       nbytes = 0;
> -               }
>
> -               /*
> -                * We 64-bit align the length of each iovec so that the start
> -                * of the next one is naturally aligned.  We'll need to
> -                * account for that slack space here. Then round nbytes up
> -                * to 64-bit alignment so that the initial buffer alignment is
> -                * easy to calculate and verify.
> -                */
> -               nbytes += niovecs * sizeof(uint64_t);
> -               nbytes = round_up(nbytes, sizeof(uint64_t));
> -
> -               /* grab the old item if it exists for reservation accounting */
> -               old_lv = lip->li_lv;
> -
> -               /*
> -                * The data buffer needs to start 64-bit aligned, so round up
> -                * that space to ensure we can align it appropriately and not
> -                * overrun the buffer.
> -                */
> -               buf_size = nbytes +
> -                          round_up((sizeof(struct xfs_log_vec) +
> -                                    niovecs * sizeof(struct xfs_log_iovec)),
> -                                   sizeof(uint64_t));
> +               /* Skip items that do not have any vectors for writing */
> +               if (!shadow->lv_niovecs && !ordered)
> +                       continue;
>
>                 /* compare to existing item size */
> -               if (lip->li_lv && buf_size <= lip->li_lv->lv_size) {
> +               if (lip->li_lv && shadow->lv_size <= lip->li_lv->lv_size) {
>                         /* same or smaller, optimise common overwrite case */
>                         lv = lip->li_lv;
>                         lv->lv_next = NULL;
> @@ -237,29 +350,28 @@ xlog_cil_insert_format_items(
>                          */
>                         *diff_iovecs -= lv->lv_niovecs;
>                         *diff_len -= lv->lv_bytes;
> +
> +                       /* Ensure the lv is set up according to ->iop_size */
> +                       lv->lv_niovecs = shadow->lv_niovecs;
> +
> +                       /* reset the lv buffer information for new formatting */
> +                       lv->lv_buf_len = 0;
> +                       lv->lv_bytes = 0;
> +                       lv->lv_buf = (char *)lv +
> +                               xlog_cil_iovec_space(lv->lv_niovecs)
>                 } else {
> -                       /* allocate new data chunk */
> -                       lv = kmem_zalloc(buf_size, KM_SLEEP|KM_NOFS);
> +                       /* switch to shadow buffer! */
> +                       lv = shadow;
>                         lv->lv_item = lip;
> -                       lv->lv_size = buf_size;
> +                       old_lv = lip->li_lv;
>                         if (ordered) {
>                                 /* track as an ordered logvec */
>                                 ASSERT(lip->li_lv == NULL);
> -                               lv->lv_buf_len = XFS_LOG_VEC_ORDERED;
>                                 goto insert;
>                         }
> -                       lv->lv_iovecp = (struct xfs_log_iovec *)&lv[1];
>                 }
>
> -               /* Ensure the lv is set up according to ->iop_size */
> -               lv->lv_niovecs = niovecs;
> -
> -               /* The allocated data region lies beyond the iovec region */
> -               lv->lv_buf_len = 0;
> -               lv->lv_bytes = 0;
> -               lv->lv_buf = (char *)lv + buf_size - nbytes;
>                 ASSERT(IS_ALIGNED((unsigned long)lv->lv_buf, sizeof(uint64_t)));
> -
>                 lip->li_ops->iop_format(lip, lv);
>  insert:
>                 ASSERT(lv->lv_buf_len <= nbytes);

Really thanks for your patch. I'll study the patch first to figure out the idea.

Gavin
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