squashfs: add optional full compressed block caching

The commit 93e72b3c612adcaca1 ("squashfs: migrate from ll_rw_block usage
to BIO") removed caching of compressed blocks in SquashFS, causing fio
performance regression in workloads with repeated file reads.  Without
caching, every read triggers disk I/O, severely impacting performance in
tools like fio.

This patch introduces a new CONFIG_SQUASHFS_COMP_CACHE_FULL Kconfig option
to enable caching of all compressed blocks, restoring performance to
pre-BIO migration levels.  When enabled, all pages in a BIO are cached in
the page cache, reducing disk I/O for repeated reads.  The fio test
results with this patch confirm the performance restoration:

For example, fio tests (iodepth=1, numjobs=1,
ioengine=psync) show a notable performance restoration:

Disable CONFIG_SQUASHFS_COMP_CACHE_FULL:
  IOPS=815, BW=102MiB/s (107MB/s)(6113MiB/60001msec)
Enable CONFIG_SQUASHFS_COMP_CACHE_FULL:
  IOPS=2223, BW=278MiB/s (291MB/s)(16.3GiB/59999msec)

The tradeoff is increased memory usage due to caching all compressed
blocks.  The CONFIG_SQUASHFS_COMP_CACHE_FULL option allows users to enable
this feature selectively, balancing performance and memory usage for
workloads with frequent repeated reads.

Link: https://lkml.kernel.org/r/20250521072559.2389-1-chanho.min@lge.com
Signed-off-by: Chanho Min <chanho.min@lge.com>
Reviewed-by Phillip Lougher <phillip@squashfs.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/fs/squashfs/Kconfig b/fs/squashfs/Kconfig
index b1091e70434a97cff69268bab9e63d412115f849..a9602aae21ef60c1ce9a100b8903f5a522f1a892 100644 (file)
--- a/fs/squashfs/Kconfig
+++ b/fs/squashfs/Kconfig
@@ -149,6 +149,27 @@ config SQUASHFS_XATTR
 
          If unsure, say N.
 
+config SQUASHFS_COMP_CACHE_FULL
+       bool "Enable full caching of compressed blocks"
+       depends on SQUASHFS
+       default n
+       help
+         This option enables caching of all compressed blocks, Without caching,
+         repeated reads of the same files trigger excessive disk I/O, significantly
+         reducinng performance in workloads like fio-based benchmarks.
+
+         For example, fio tests (iodepth=1, numjobs=1, ioengine=psync) show:
+          With caching: IOPS=2223, BW=278MiB/s (291MB/s)
+          Without caching: IOPS=815, BW=102MiB/s (107MB/s)
+
+         Enabling this option restores performance to pre-regression levels by
+         caching all compressed blocks in the page cache, reducing disk I/O for
+         repeated reads. However, this increases memory usage, which may be a
+         concern in memory-constrained environments.
+
+         Enable this option if your workload involves frequent repeated reads and
+         memory usage is not a limiting factor. If unsure, say N.
+
 config SQUASHFS_ZLIB
        bool "Include support for ZLIB compressed file systems"
        depends on SQUASHFS

diff --git a/fs/squashfs/block.c b/fs/squashfs/block.c
index 2dc730800f448d8cb44f2d5c4e625e607d1faf2f..3061043e915c8775f0498083f3219e57c0320bf2 100644 (file)
--- a/fs/squashfs/block.c
+++ b/fs/squashfs/block.c
@@ -88,6 +88,10 @@ static int squashfs_bio_read_cached(struct bio *fullbio,
        struct bio_vec *bv;
        int idx = 0;
        int err = 0;
+#ifdef CONFIG_SQUASHFS_COMP_CACHE_FULL
+       struct page **cache_pages = kmalloc_array(page_count,
+                       sizeof(void *), GFP_KERNEL | __GFP_ZERO);
+#endif
 
        bio_for_each_segment_all(bv, fullbio, iter_all) {
                struct page *page = bv->bv_page;
@@ -110,6 +114,11 @@ static int squashfs_bio_read_cached(struct bio *fullbio,
                        head_to_cache = page;
                else if (idx == page_count - 1 && index + length != read_end)
                        tail_to_cache = page;
+#ifdef CONFIG_SQUASHFS_COMP_CACHE_FULL
+               /* Cache all pages in the BIO for repeated reads */
+               else if (cache_pages)
+                       cache_pages[idx] = page;
+#endif
 
                if (!bio || idx != end_idx) {
                        struct bio *new = bio_alloc_clone(bdev, fullbio,
@@ -163,6 +172,25 @@ static int squashfs_bio_read_cached(struct bio *fullbio,
                }
        }
 
+#ifdef CONFIG_SQUASHFS_COMP_CACHE_FULL
+       if (!cache_pages)
+               goto out;
+
+       for (idx = 0; idx < page_count; idx++) {
+               if (!cache_pages[idx])
+                       continue;
+               int ret = add_to_page_cache_lru(cache_pages[idx], cache_mapping,
+                                               (read_start >> PAGE_SHIFT) + idx,
+                                               GFP_NOIO);
+
+               if (!ret) {
+                       SetPageUptodate(cache_pages[idx]);
+                       unlock_page(cache_pages[idx]);
+               }
+       }
+       kfree(cache_pages);
+out:
+#endif
        return 0;
 }