• Phillip Lougher's avatar
    squashfs: extend "page actor" to handle missing pages · f268eedd
    Phillip Lougher authored
    Patch series "Squashfs: handle missing pages decompressing into page
    cache".
    
    This patchset enables Squashfs to handle missing pages when directly
    decompressing datablocks into the page cache.
    
    Previously if the full set of pages needed was not available, Squashfs
    would have to fall back to using an intermediate buffer (the older
    method), which is slower, involving a memcopy, and it introduces
    contention on a shared buffer.
    
    The first patch extends the "page actor" code to handle missing pages.
    
    The second patch updates Squashfs_readpage_block() to use the new
    functionality, and removes the code that falls back to using an
    intermediate buffer.
    
    This patchset is independent of the readahead work, and it is standalone. 
    It can be merged on its own.
    
    But the readahead patch for efficiency also needs this patch-set.
    
    
    This patch (of 2):
    
    This patch extends the "page actor" code to handle missing pages.
    
    Previously if the full set of pages needed to decompress a Squashfs
    datablock was unavailable, this would cause decompression to fail on the
    missing pages.
    
    In this case direct decompression into the page cache could not be
    achieved and the code would fall back to using the older intermediate
    buffer method.
    
    With this patch, direct decompression into the page cache can be achieved
    with missing pages.
    
    For "multi-shot" decompressors (zlib, xz, zstd), the page actor will
    allocate a temporary buffer which is passed to the decompressor, and then
    freed by the page actor.
    
    For "single shot" decompressors (lz4, lzo) which decompress into a
    contiguous "bounce buffer", and which is then copied into the page cache,
    it would be pointless to allocate a temporary buffer, memcpy into it, and
    then free it.  For these decompressors -ENOMEM is returned, which
    signifies that the memcpy for that page should be skipped.
    
    This also happens if the data block is uncompressed.
    
    Link: https://lkml.kernel.org/r/20220611032133.5743-1-phillip@squashfs.org.uk
    Link: https://lkml.kernel.org/r/20220611032133.5743-2-phillip@squashfs.org.ukSigned-off-by: default avatarPhillip Lougher <phillip@squashfs.org.uk>
    Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
    Cc: Hsin-Yi Wang <hsinyi@chromium.org>
    Cc: Xiongwei Song <Xiongwei.Song@windriver.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    f268eedd
block.c 5.85 KB