RE: [f2fs-dev] [PATCH 1/2] f2fs: handle failed bio allocation

[Chao Yu]

> -----Original Message-----
> From: Jaegeuk Kim [mailto:jaegeuk@xxxxxxxxxx]
> Sent: Thursday, August 20, 2015 11:57 PM
> To: Chao Yu
> Cc: linux-kernel@xxxxxxxxxxxxxxx; linux-fsdevel@xxxxxxxxxxxxxxx;
> linux-f2fs-devel@xxxxxxxxxxxxxxxxxxxxx
> Subject: Re: [f2fs-dev] [PATCH 1/2] f2fs: handle failed bio allocation
> 
> On Thu, Aug 20, 2015 at 05:08:24PM +0800, Chao Yu wrote:
> > Hi Jaegeuk,
> >
> > > -----Original Message-----
> > > From: Jaegeuk Kim [mailto:jaegeuk@xxxxxxxxxx]
> > > Sent: Saturday, August 15, 2015 7:09 AM
> > > To: linux-kernel@xxxxxxxxxxxxxxx; linux-fsdevel@xxxxxxxxxxxxxxx;
> > > linux-f2fs-devel@xxxxxxxxxxxxxxxxxxxxx
> > > Cc: Jaegeuk Kim
> > > Subject: [f2fs-dev] [PATCH 1/2] f2fs: handle failed bio allocation
> > >
> > > As the below comment of bio_alloc_bioset, f2fs can allocate multiple bios at the
> > > same time. So, we can't guarantee that bio is allocated all the time.
> > >
> > > "
> > >  *   When @bs is not NULL, if %__GFP_WAIT is set then bio_alloc will always be
> > >  *   able to allocate a bio. This is due to the mempool guarantees. To make this
> > >  *   work, callers must never allocate more than 1 bio at a time from this pool.
> > >  *   Callers that need to allocate more than 1 bio must always submit the
> > >  *   previously allocated bio for IO before attempting to allocate a new one.
> > >  *   Failure to do so can cause deadlocks under memory pressure.
> > > "
> > >
> > > Signed-off-by: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
> > > ---
> > >  fs/f2fs/data.c    |  3 +--
> > >  fs/f2fs/f2fs.h    | 15 +++++++++++++++
> > >  fs/f2fs/segment.c | 14 +++++++++++---
> > >  3 files changed, 27 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> > > index cad9ebe..726e58b 100644
> > > --- a/fs/f2fs/data.c
> > > +++ b/fs/f2fs/data.c
> > > @@ -90,8 +90,7 @@ static struct bio *__bio_alloc(struct f2fs_sb_info *sbi, block_t blk_addr,
> > >  {
> > >  	struct bio *bio;
> > >
> > > -	/* No failure on bio allocation */
> > > -	bio = bio_alloc(GFP_NOIO, npages);
> >
> > How about using __GFP_NOFAIL flag to avoid failing in bio_alloc instead
> > of adding opencode endless loop in code?
> >
> > We can see the reason in this commit 	647757197cd3
> > ("mm: clarify __GFP_NOFAIL deprecation status ")
> >
> > "__GFP_NOFAIL is documented as a deprecated flag since commit
> > 478352e789f5 ("mm: add comment about deprecation of __GFP_NOFAIL").
> >
> > This has discouraged people from using it but in some cases an opencoded
> > endless loop around allocator has been used instead. So the allocator
> > is not aware of the de facto __GFP_NOFAIL allocation because this
> > information was not communicated properly.
> >
> > Let's make clear that if the allocation context really cannot afford
> > failure because there is no good failure policy then using __GFP_NOFAIL
> > is preferable to opencoding the loop outside of the allocator."
> >
> > BTW, I found that f2fs_kmem_cache_alloc also could be replaced, we could
> > fix them together.
> 
> Agreed. I think that can be another patch like this.
> 
> From 1579e0d1ada96994c4ec6619fb5b5d9386e77ab3 Mon Sep 17 00:00:00 2001
> From: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
> Date: Thu, 20 Aug 2015 08:51:56 -0700
> Subject: [PATCH] f2fs: use __GFP_NOFAIL to avoid infinite loop
> 
> __GFP_NOFAIL can avoid retrying the whole path of kmem_cache_alloc and
> bio_alloc.
> 
> Suggested-by: Chao Yu <chao2.yu@xxxxxxxxxxx>
> Signed-off-by: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
> ---
>  fs/f2fs/f2fs.h | 16 +++++-----------
>  1 file changed, 5 insertions(+), 11 deletions(-)
> 
> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> index 00591f7..c78b599 100644
> --- a/fs/f2fs/f2fs.h
> +++ b/fs/f2fs/f2fs.h
> @@ -1244,13 +1244,10 @@ static inline void *f2fs_kmem_cache_alloc(struct kmem_cache *cachep,
>  						gfp_t flags)
>  {
>  	void *entry;
> -retry:
> -	entry = kmem_cache_alloc(cachep, flags);
> -	if (!entry) {
> -		cond_resched();
> -		goto retry;
> -	}
> 
> +	entry = kmem_cache_alloc(cachep, flags);
> +	if (!entry)
> +		entry = kmem_cache_alloc(cachep, flags | __GFP_NOFAIL);

The fast + slow path model looks good to me, expect one thing:
In several paths of checkpoint, caller will grab slab cache with GFP_ATOMIC,
so in slow path, our flags will be GFP_ATOMIC | __GFP_NOFAIL, I'm not sure
that the two flags can be used together.

Should we replace GFP_ATOMIC with GFP_NOFS in flags if caller passed
GFP_ATOMIC?

Thanks,

>  	return entry;
>  }
> 
> @@ -1259,12 +1256,9 @@ static inline struct bio *f2fs_bio_alloc(int npages)
>  	struct bio *bio;
> 
>  	/* No failure on bio allocation */
> -retry:
>  	bio = bio_alloc(GFP_NOIO, npages);
> -	if (!bio) {
> -		cond_resched();
> -		goto retry;
> -	}
> +	if (!bio)
> +		bio = bio_alloc(GFP_NOIO | __GFP_NOFAIL, npages);
>  	return bio;
>  }
> 
> --
> 2.1.1

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