Re: [RFC] Better page cache error handling

[Jan Kara]

Hi Matthew!

On Fri 05-02-21 16:11:42, Matthew Wilcox wrote:
> Scenario:
> 
> You have a disk with a bad sector.  This disk will take 30 seconds of
> trying progressively harder to recover the sector before timing the
> I/O out and returning BLK_STS_MEDIUM to the filesystem.  Unfortunately
> for you, this bad sector happens to have landed in index.html on your
> webserver which gets one hit per second.  You have configured 500
> threads on your webserver.
> 
> Today:
> 
> We allocate a page and try to read it.  29 threads pile up waiting
> for the page lock in filemap_update_page() (or whatever variant of
> that you're looking at; I'm talking about linux-next).  The original
> requester gets the error and returns -EIO to userspace.  One of the
> lucky 29 waiting threads sends another read.  30 more threads pile up
> while it's processing.  Eventually, all 500 threads of your webserver
> are sleeping waiting for their turn to get an EIO.
> 
> With the below patch:
> 
> We allocate a page and try to read it.  29 threads pile up waiting
> for the page lock in filemap_update_page().  The error returned by the
> original I/O is shared between all 29 waiters as well as being returned
> to the requesting thread.  The next request for index.html will send
> another I/O, and more waiters will pile up trying to get the page lock,
> but at no time will more than 30 threads be waiting for the I/O to fail.

Interesting idea. It certainly improves current behavior. I just wonder
whether this isn't a partial solution to a problem and a full solution of
it would have to go in a different direction? I mean it just seems
wrong that each reader (let's assume they just won't overlap) has to retry
the failed IO and wait for the HW to figure out it's not going to work.
Shouldn't we cache the error state with the page? And I understand that we
then also have to deal with the problem how to invalidate the error state
when the block might eventually become readable (for stuff like temporary
IO failures). That would need some signalling from the driver to the page
cache, maybe in a form of some error recovery sequence counter or something
like that. For stuff like iSCSI, multipath, or NBD it could be doable I
believe...

								Honza

> 
> ----
> 
> Individual filesystems will have to be modified to call unlock_page_err()
> to take advantage of this.  Unconverted filesystems will continue to
> behave as in the first scenario.
> 
> I've only tested it lightly, but it doesn't seem to break anything.
> It needs some targetted testing with error injection which I haven't
> done yet.  It probably also needs some refinement to not report
> errors from readahead.  Also need to audit the other callers of
> put_and_wait_on_page_locked().  This patch interferes with the page
> folio work, so I'm not planning on pushing it for a couple of releases.
> 
> ----
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index 16a1e82e3aeb..faeb6c4af7fd 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -183,7 +183,7 @@ iomap_read_page_end_io(struct bio_vec *bvec, int error)
>  	}
>  
>  	if (!iop || atomic_sub_and_test(bvec->bv_len, &iop->read_bytes_pending))
> -		unlock_page(page);
> +		unlock_page_err(page, error);
>  }
>  
>  static void
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index fda84e88b2ba..e750881bedfe 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -564,11 +564,13 @@ struct wait_page_key {
>  	struct page *page;
>  	int bit_nr;
>  	int page_match;
> +	int err;
>  };
>  
>  struct wait_page_queue {
>  	struct page *page;
>  	int bit_nr;
> +	int err;
>  	wait_queue_entry_t wait;
>  };
>  
> @@ -591,6 +593,7 @@ extern int __lock_page_async(struct page *page, struct wait_page_queue *wait);
>  extern int __lock_page_or_retry(struct page *page, struct mm_struct *mm,
>  				unsigned int flags);
>  extern void unlock_page(struct page *page);
> +extern void unlock_page_err(struct page *page, int err);
>  extern void unlock_page_fscache(struct page *page);
>  
>  /*
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 97ff7294516e..515e0136f00f 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1077,6 +1077,7 @@ static int wake_page_function(wait_queue_entry_t *wait, unsigned mode, int sync,
>  	 * afterwards to avoid any races. This store-release pairs
>  	 * with the load-acquire in wait_on_page_bit_common().
>  	 */
> +	wait_page->err = key->err;
>  	smp_store_release(&wait->flags, flags | WQ_FLAG_WOKEN);
>  	wake_up_state(wait->private, mode);
>  
> @@ -1094,7 +1095,7 @@ static int wake_page_function(wait_queue_entry_t *wait, unsigned mode, int sync,
>  	return (flags & WQ_FLAG_EXCLUSIVE) != 0;
>  }
>  
> -static void wake_up_page_bit(struct page *page, int bit_nr)
> +static void wake_up_page_bit(struct page *page, int bit_nr, int err)
>  {
>  	wait_queue_head_t *q = page_waitqueue(page);
>  	struct wait_page_key key;
> @@ -1103,6 +1104,7 @@ static void wake_up_page_bit(struct page *page, int bit_nr)
>  
>  	key.page = page;
>  	key.bit_nr = bit_nr;
> +	key.err = err;
>  	key.page_match = 0;
>  
>  	bookmark.flags = 0;
> @@ -1152,7 +1154,7 @@ static void wake_up_page(struct page *page, int bit)
>  {
>  	if (!PageWaiters(page))
>  		return;
> -	wake_up_page_bit(page, bit);
> +	wake_up_page_bit(page, bit, 0);
>  }
>  
>  /*
> @@ -1214,6 +1216,7 @@ static inline int wait_on_page_bit_common(wait_queue_head_t *q,
>  	wait->func = wake_page_function;
>  	wait_page.page = page;
>  	wait_page.bit_nr = bit_nr;
> +	wait_page.err = 0;
>  
>  repeat:
>  	wait->flags = 0;
> @@ -1325,8 +1328,10 @@ static inline int wait_on_page_bit_common(wait_queue_head_t *q,
>  	 */
>  	if (behavior == EXCLUSIVE)
>  		return wait->flags & WQ_FLAG_DONE ? 0 : -EINTR;
> +	if (behavior != DROP)
> +		wait_page.err = 0;
>  
> -	return wait->flags & WQ_FLAG_WOKEN ? 0 : -EINTR;
> +	return wait->flags & WQ_FLAG_WOKEN ? wait_page.err : -EINTR;
>  }
>  
>  void wait_on_page_bit(struct page *page, int bit_nr)
> @@ -1408,8 +1413,9 @@ static inline bool clear_bit_unlock_is_negative_byte(long nr, volatile void *mem
>  #endif
>  
>  /**
> - * unlock_page - unlock a locked page
> + * unlock_page_err - unlock a locked page
>   * @page: the page
> + * @err: errno to be communicated to waiters
>   *
>   * Unlocks the page and wakes up sleepers in wait_on_page_locked().
>   * Also wakes sleepers in wait_on_page_writeback() because the wakeup
> @@ -1422,13 +1428,19 @@ static inline bool clear_bit_unlock_is_negative_byte(long nr, volatile void *mem
>   * portably (architectures that do LL/SC can test any bit, while x86 can
>   * test the sign bit).
>   */
> -void unlock_page(struct page *page)
> +void unlock_page_err(struct page *page, int err)
>  {
>  	BUILD_BUG_ON(PG_waiters != 7);
>  	page = compound_head(page);
>  	VM_BUG_ON_PAGE(!PageLocked(page), page);
>  	if (clear_bit_unlock_is_negative_byte(PG_locked, &page->flags))
> -		wake_up_page_bit(page, PG_locked);
> +		wake_up_page_bit(page, PG_locked, err);
> +}
> +EXPORT_SYMBOL(unlock_page_err);
> +
> +void unlock_page(struct page *page)
> +{
> +	unlock_page_err(page, 0);
>  }
>  EXPORT_SYMBOL(unlock_page);
>  
> @@ -1446,7 +1458,7 @@ void unlock_page_fscache(struct page *page)
>  	page = compound_head(page);
>  	VM_BUG_ON_PAGE(!PagePrivate2(page), page);
>  	clear_bit_unlock(PG_fscache, &page->flags);
> -	wake_up_page_bit(page, PG_fscache);
> +	wake_up_page_bit(page, PG_fscache, 0);
>  }
>  EXPORT_SYMBOL(unlock_page_fscache);
>  
> @@ -2298,8 +2310,11 @@ static int filemap_update_page(struct kiocb *iocb,
>  		if (iocb->ki_flags & (IOCB_NOWAIT | IOCB_NOIO))
>  			return -EAGAIN;
>  		if (!(iocb->ki_flags & IOCB_WAITQ)) {
> -			put_and_wait_on_page_locked(page, TASK_KILLABLE);
> -			return AOP_TRUNCATED_PAGE;
> +			error = put_and_wait_on_page_locked(page,
> +					TASK_KILLABLE);
> +			if (!error)
> +				return AOP_TRUNCATED_PAGE;
> +			return error;
>  		}
>  		error = __lock_page_async(page, iocb->ki_waitq);
>  		if (error)
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR