mm: zswap: use object read/write APIs instead of object mapping APIs

Use the new object read/write APIs instead of mapping APIs.

On compress side, zpool_obj_write() is more concise and provides exactly
what zswap needs to write the compressed object to the zpool, instead of
map->copy->unmap.

On the decompress side, zpool_obj_read_begin() is sleepable, which
allows avoiding the memcpy() for zsmalloc and slightly simplifying the
code by:
- Avoiding checking if the zpool driver is sleepable, reducing special
  cases and shrinking the huge comment.
- Having a single zpool_obj_read_end() call rather than multiple
  conditional zpool_unmap_handle() calls.

The !virt_addr_valid() case can be removed in the future if the crypto API
supports kmap addresses or by using kmap_to_page(), completely eliminating
the memcpy() path in zswap_decompress().  This a step toward that.  In
that spirit, opportunistically make the comment more specific about the
kmap case instead of generic non-linear addresses.  This is the only case
that needs to be handled in practice, and the generic comment makes it
seem like a bigger problem that it actually is.

Link: https://lkml.kernel.org/r/20250305061134.4105762-3-yosry.ahmed@linux.dev
Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Nhat Pham <nphamcs@gmail.com>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/zswap.c b/mm/zswap.c
index 8a1ded8fa97310c5f77c394b8f27ad14225a0ac4..7de54f105d04db7ba181569452ded1ee9432f5e8 100644 (file)
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -930,7 +930,6 @@ static bool zswap_compress(struct page *page, struct zswap_entry *entry,
        unsigned int dlen = PAGE_SIZE;
        unsigned long handle;
        struct zpool *zpool;
-       char *buf;
        gfp_t gfp;
        u8 *dst;
 
@@ -972,10 +971,7 @@ static bool zswap_compress(struct page *page, struct zswap_entry *entry,
        if (alloc_ret)
                goto unlock;
 
-       buf = zpool_map_handle(zpool, handle, ZPOOL_MM_WO);
-       memcpy(buf, dst, dlen);
-       zpool_unmap_handle(zpool, handle);
-
+       zpool_obj_write(zpool, handle, dst, dlen);
        entry->handle = handle;
        entry->length = dlen;
 
@@ -996,24 +992,22 @@ static void zswap_decompress(struct zswap_entry *entry, struct folio *folio)
        struct zpool *zpool = entry->pool->zpool;
        struct scatterlist input, output;
        struct crypto_acomp_ctx *acomp_ctx;
-       u8 *src;
+       u8 *src, *obj;
 
        acomp_ctx = acomp_ctx_get_cpu_lock(entry->pool);
-       src = zpool_map_handle(zpool, entry->handle, ZPOOL_MM_RO);
+       obj = zpool_obj_read_begin(zpool, entry->handle, acomp_ctx->buffer);
+
        /*
-        * If zpool_map_handle is atomic, we cannot reliably utilize its mapped buffer
-        * to do crypto_acomp_decompress() which might sleep. In such cases, we must
-        * resort to copying the buffer to a temporary one.
-        * Meanwhile, zpool_map_handle() might return a non-linearly mapped buffer,
-        * such as a kmap address of high memory or even ever a vmap address.
-        * However, sg_init_one is only equipped to handle linearly mapped low memory.
-        * In such cases, we also must copy the buffer to a temporary and lowmem one.
+        * zpool_obj_read_begin() might return a kmap address of highmem when
+        * acomp_ctx->buffer is not used.  However, sg_init_one() does not
+        * handle highmem addresses, so copy the object to acomp_ctx->buffer.
         */
-       if ((acomp_ctx->is_sleepable && !zpool_can_sleep_mapped(zpool)) ||
-           !virt_addr_valid(src)) {
-               memcpy(acomp_ctx->buffer, src, entry->length);
+       if (virt_addr_valid(obj)) {
+               src = obj;
+       } else {
+               WARN_ON_ONCE(obj == acomp_ctx->buffer);
+               memcpy(acomp_ctx->buffer, obj, entry->length);
                src = acomp_ctx->buffer;
-               zpool_unmap_handle(zpool, entry->handle);
        }
 
        sg_init_one(&input, src, entry->length);
@@ -1023,8 +1017,7 @@ static void zswap_decompress(struct zswap_entry *entry, struct folio *folio)
        BUG_ON(crypto_wait_req(crypto_acomp_decompress(acomp_ctx->req), &acomp_ctx->wait));
        BUG_ON(acomp_ctx->req->dlen != PAGE_SIZE);
 
-       if (src != acomp_ctx->buffer)
-               zpool_unmap_handle(zpool, entry->handle);
+       zpool_obj_read_end(zpool, entry->handle, obj);
        acomp_ctx_put_unlock(acomp_ctx);
 }