Re: [PATCH 1/5] mm/zswap: reuse dstmem when decompress

[Yosry Ahmed <yosryahmed@google.com>]

On Thu, Dec 14, 2023 at 5:29 AM Chengming Zhou
<zhouchengming@bytedance.com> wrote:
>
> On 2023/12/14 07:24, Yosry Ahmed wrote:
> > On Tue, Dec 12, 2023 at 8:18 PM Chengming Zhou
> > <zhouchengming@bytedance.com> wrote:
> >>
> >> In the !zpool_can_sleep_mapped() case such as zsmalloc, we need to first
> >> copy the entry->handle memory to a temporary memory, which is allocated
> >> using kmalloc.
> >>
> >> Obviously we can reuse the per-compressor dstmem to avoid allocating
> >> every time, since it's percpu-compressor and protected in mutex.
> >>
> >> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
> >> Reviewed-by: Nhat Pham <nphamcs@gmail.com>
> >> ---
> >>  mm/zswap.c | 29 +++++++++--------------------
> >>  1 file changed, 9 insertions(+), 20 deletions(-)
> >>
> >> diff --git a/mm/zswap.c b/mm/zswap.c
> >> index 7ee54a3d8281..edb8b45ed5a1 100644
> >> --- a/mm/zswap.c
> >> +++ b/mm/zswap.c
> >> @@ -1772,9 +1772,9 @@ bool zswap_load(struct folio *folio)
> >>         struct zswap_entry *entry;
> >>         struct scatterlist input, output;
> >>         struct crypto_acomp_ctx *acomp_ctx;
> >> -       u8 *src, *dst, *tmp;
> >> +       unsigned int dlen = PAGE_SIZE;
> >> +       u8 *src, *dst;
> >>         struct zpool *zpool;
> >> -       unsigned int dlen;
> >>         bool ret;
> >>
> >>         VM_WARN_ON_ONCE(!folio_test_locked(folio));
> >> @@ -1796,27 +1796,18 @@ bool zswap_load(struct folio *folio)
> >>                 goto stats;
> >>         }
> >>
> >> -       zpool = zswap_find_zpool(entry);
> >> -       if (!zpool_can_sleep_mapped(zpool)) {
> >> -               tmp = kmalloc(entry->length, GFP_KERNEL);
> >> -               if (!tmp) {
> >> -                       ret = false;
> >> -                       goto freeentry;
> >> -               }
> >> -       }
> >> -
> >>         /* decompress */
> >> -       dlen = PAGE_SIZE;
> >> -       src = zpool_map_handle(zpool, entry->handle, ZPOOL_MM_RO);
> >> +       acomp_ctx = raw_cpu_ptr(entry->pool->acomp_ctx);
> >> +       mutex_lock(acomp_ctx->mutex);
> >>
> >> +       zpool = zswap_find_zpool(entry);
> >> +       src = zpool_map_handle(zpool, entry->handle, ZPOOL_MM_RO);
> >>         if (!zpool_can_sleep_mapped(zpool)) {
> >> -               memcpy(tmp, src, entry->length);
> >> -               src = tmp;
> >> +               memcpy(acomp_ctx->dstmem, src, entry->length);
> >> +               src = acomp_ctx->dstmem;
> >
> > I don't like that we are now using acomp_ctx->dstmem and
> > acomp_ctx->mutex now for purposes other than what the naming suggests.
>
> The "mutex" name is coherent, "dstmem" depends on how we use IMHO.
> Change to just "mem"? Or do you have a better name to replace?
>
> >
> > How about removing these two fields from acomp_ctx, and directly using
> > zswap_dstmem and zswap_mutex in both the load and store paths, rename
> > them, and add proper comments above their definitions that they are
> > for generic percpu buffering on the load and store paths?
>
> Yes, they are percpu memory and lock, but they are used by per acomp_ctx,
> and the cpu maybe changing in the middle, so maybe better to keep them.

I don't mean to remove completely. Keep them as (for example)
zswap_mem and zswap_mutex global percpu variables, and not have
pointers in acomp_ctx to them. Instead of using acomp_ctx->dstmem
today, we directly use the global zswap_mem (same for the mutex).

This makes it clear that the buffers are not owned or exclusively used
by the acomp_ctx. WDYT?