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

Jaegeuk Kim
2015-08-20T15:57:32Z

On Thu, Aug 20, 2015 at 05:08:24PM +0800, Chao Yu wrote:
> Hi Jaegeuk,
>
> > -----Original Message-----
> > From: Jaegeuk Kim [mailto:jaegeuk@kernel.org]
> > Sent: Saturday, August 15, 2015 7:09 AM
> > To: linux-kernel@vger.kernel.org; linux-fsdevel@vger.kernel.org;
> > linux-f2fs-devel@lists.sourceforge.net
> > 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@kernel.org>
> > ---
> > 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@kernel.org>
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@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
---
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);
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