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    SubjectRe: [PATCH 3/7] mm/zswap: reuse dstmem when decompress
    On Wed, Dec 6, 2023 at 1:46 AM 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.

    Ah this sounds like a good idea. We have to grab that mutex anyway -
    might as well use the memory slot that is protected by that mutex.

    >
    > Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
    > ---
    > mm/zswap.c | 29 +++++++++--------------------
    > 1 file changed, 9 insertions(+), 20 deletions(-)
    >
    > diff --git a/mm/zswap.c b/mm/zswap.c
    > index a6b4859a0164..d93a7b58b5af 100644
    > --- a/mm/zswap.c
    > +++ b/mm/zswap.c
    > @@ -1758,9 +1758,9 @@ 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));
    > @@ -1782,27 +1782,18 @@ 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;

    We're moving handle (un)mapping and the memory copying inside the
    critical section protected by the mutex. Seems fine to me -
    zswap_store() already did this IIUC.

    > zpool_unmap_handle(zpool, entry->handle);
    > }
    >
    > - acomp_ctx = raw_cpu_ptr(entry->pool->acomp_ctx);
    > - mutex_lock(acomp_ctx->mutex);
    > sg_init_one(&input, src, entry->length);
    > sg_init_table(&output, 1);
    > sg_set_page(&output, page, PAGE_SIZE, 0);
    > @@ -1813,15 +1804,13 @@ bool zswap_load(struct folio *folio)
    >
    > if (zpool_can_sleep_mapped(zpool))
    > zpool_unmap_handle(zpool, entry->handle);
    > - else
    > - kfree(tmp);
    >
    > ret = true;
    > stats:
    > count_vm_event(ZSWPIN);
    > if (entry->objcg)
    > count_objcg_event(entry->objcg, ZSWPIN);
    > -freeentry:

    So it reduces the chance of zswap_load() failure due to unable to
    memory allocation failure? Nice!

    > +
    > spin_lock(&tree->lock);
    > if (ret && zswap_exclusive_loads_enabled) {
    > zswap_invalidate_entry(tree, entry);
    >
    > --
    > b4 0.10.1

    Reviewed-by: Nhat Pham <nphamcs@gmail.com>

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