linux-kernel - Re: [PATCH] mm: zswap: increase reject_compress_poor but not reject_compress

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Date: Fri, 16 Feb 2024 01:17:12 -0800
From: Nhat Pham <nphamcs@...il.com>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: Barry Song <21cnbao@...il.com>, akpm@...ux-foundation.org, hannes@...xchg.org, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	Barry Song <v-songbaohua@...o.com>, Chengming Zhou <zhouchengming@...edance.com>, 
	Sergey Senozhatsky <senozhatsky@...omium.org>
Subject: Re: [PATCH] mm: zswap: increase reject_compress_poor but not
 reject_compress_fail if compression returns ENOSPC

On Fri, Feb 16, 2024 at 12:23 AM Yosry Ahmed <yosryahmed@...gle.com> wrote:
>
> On Fri, Feb 16, 2024 at 04:05:39PM +1300, Barry Song wrote:
> > From: Barry Song <v-songbaohua@...o.com>
> >
> > My commit fc8580edbaa6 ("mm: zsmalloc: return -ENOSPC rather than -EINVAL
> > in zs_malloc while size is too large") wanted to depend on zs_malloc's
> > returned ENOSPC to distinguish the case that compressed data is larger
> > than the original data from normal compression cases. The commit, for
> > sure, was correct and worked as expected but the code wouldn't run to
> > there after commit 744e1885922a ("crypto: scomp - fix req->dst buffer
> > overflow") as Chengming's this patch makes zswap_store() goto out
> > immediately after the special compression case happens. So there is
> > no chance to execute zs_malloc() now. We need to fix the count right
> > after compressions return ENOSPC.
> >
> > Fixes: fc8580edbaa6 ("mm: zsmalloc: return -ENOSPC rather than -EINVAL in zs_malloc while size is too large")
>
> I don't see how this is a fix for that commit. Commit fc8580edbaa6 made
> sure zsmalloc returns a correct errno when the compressed size is too
> large. The fact that zswap stores were failing before calling into
> zsmalloc and not reporting the error correctly in debug counters is not
> that commits fault.
>
> I think the proper fixes should be 744e1885922a if it introduced the
> first scenario where -ENOSPC can be returned from scomp without handling
> it properly in zswap. If -ENOSPC was a possible return value before
> that, then it should be cb61dad80fdc ("zswap: export compression failure
> stats"), where the counter was introduced.

IIRC, the counter was introduced before the zsmalloc patch that
allowed for returning -ENOSPC, as well as the patch that allowed
crypto API to return -ENOSPC.

I think "Fixes: 744e1885922a" would be the closest, as it introduces
the -ENOSPC return value, without handling it in zswap_store().


>
> > Cc: Chengming Zhou <zhouchengming@...edance.com>
> > Cc: Nhat Pham <nphamcs@...il.com>
> > Cc: Sergey Senozhatsky <senozhatsky@...omium.org>
> > Signed-off-by: Barry Song <v-songbaohua@...o.com>
> > ---
> >  mm/zswap.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/zswap.c b/mm/zswap.c
> > index 6319d2281020..9a21dbe8c056 100644
> > --- a/mm/zswap.c
> > +++ b/mm/zswap.c
> > @@ -1627,7 +1627,10 @@ bool zswap_store(struct folio *folio)
> >       dlen = acomp_ctx->req->dlen;
> >
> >       if (ret) {
> > -             zswap_reject_compress_fail++;
> > +             if (ret == -ENOSPC)
> > +                     zswap_reject_compress_poor++;
> > +             else
> > +                     zswap_reject_compress_fail++;
>
> With this diff, we have four locations in zswap_store() where we
> increment zswap_reject_compress_{poor/fail}.
>
> How about the following instead?A
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 62fe307521c93..3a7e8ba7f6116 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -1059,24 +1059,16 @@ static bool zswap_compress(struct folio *folio, struct zswap_entry *entry)
>          */
>         ret = crypto_wait_req(crypto_acomp_compress(acomp_ctx->req), &acomp_ctx->wait);
>         dlen = acomp_ctx->req->dlen;
> -       if (ret) {
> -               zswap_reject_compress_fail++;
> +       if (ret)
>                 goto unlock;
> -       }
>
>         zpool = zswap_find_zpool(entry);
>         gfp = __GFP_NORETRY | __GFP_NOWARN | __GFP_KSWAPD_RECLAIM;
>         if (zpool_malloc_support_movable(zpool))
>                 gfp |= __GFP_HIGHMEM | __GFP_MOVABLE;
>         ret = zpool_malloc(zpool, dlen, gfp, &handle);
> -       if (ret == -ENOSPC) {
> -               zswap_reject_compress_poor++;
> -               goto unlock;
> -       }
> -       if (ret) {
> -               zswap_reject_alloc_fail++;
> +       if (ret)
>                 goto unlock;
> -       }
>
>         buf = zpool_map_handle(zpool, handle, ZPOOL_MM_WO);
>         memcpy(buf, dst, dlen);
> @@ -1086,6 +1078,10 @@ static bool zswap_compress(struct folio *folio, struct zswap_entry *entry)
>         entry->length = dlen;
>
>  unlock:
> +       if (ret == -ENOSPC)
> +               zswap_reject_compress_poor++;
> +       else if (ret)
> +               zswap_reject_alloc_fail++;

I'm eyeballing this, but we have 3 debug counters possible right?
zswap_reject_compress_poor, zswap_reject_compress_fail,
zswap_reject_alloc_fail. I think you remove 3 incrementations (is that
a word lol), and add only 2 cases here.

>         mutex_unlock(&acomp_ctx->mutex);
>         return ret == 0;
>  }
>
> >               goto put_dstmem;
> >       }
> >
> > --
> > 2.34.1
> >