On 15/07/2025 21:40, Matthew Wilcox wrote:
I've started looking at how the page cache can help filesystems handle
compressed data better. Feedback would be appreciated! I'll probably
say a few things which are obvious to anyone who knows how compressed
files work, but I'm trying to be explicit about my assumptions.
First, I believe that all filesystems work by compressing fixed-size
plaintext into variable-sized compressed blocks. This would be a good
point to stop reading and tell me about counterexamples.
For Squashfs Yes.
From what I've been reading in all your filesystems is that you want to
allocate extra pages in the page cache in order to store the excess data
retrieved along with the page that you're actually trying to read. That's
because compressing in larger chunks leads to better compression.
Yes.
There's some discrepancy between filesystems whether you need scratch
space for decompression. Some filesystems read the compressed data into
the pagecache and decompress in-place, while other filesystems read the
compressed data into scratch pages and decompress into the page cache.
Squashfs uses scratch pages.
There also seems to be some discrepancy between filesystems whether the
decompression involves vmap() of all the memory allocated or whether the
decompression routines can handle doing kmap_local() on individual pages.
Squashfs does both, and this depends on whether the decompression
algorithm implementation in the kernel is multi-shot or single-shot.
The zlib/xz/zstd decompressors are multi-shot, in that you can call them
multiply, giving them an extra input or output buffer when it runs out.
This means you can get them to output into a 4K page at a time, without
requiring the pages to be contiguous. kmap_local() can be called on each
page before passing it to the decompressor.
The lzo/lz4 decompressors are single-shot, they expect to be called once,
with a single contiguous input buffer containing the data to be
decompressed, and a single contiguous output buffer large enough to hold
all the uncompressed data.
So, my proposal is that filesystems tell the page cache that their minimum
folio size is the compression block size. That seems to be around 64k,
so not an unreasonable minimum allocation size. That removes all the
extra code in filesystems to allocate extra memory in the page cache.
It means we don't attempt to track dirtiness at a sub-folio granularity
(there's no point, we have to write back the entire compressed bock
at once). We also get a single virtually contiguous block ... if you're
willing to ditch HIGHMEM support. Or there's a proposal to introduce a
vmap_file() which would give us a virtually contiguous chunk of memory
(and could be trivially turned into a noop for the case of trying to
vmap a single large folio).
The compression block size in Squashfs can be 4K to 1M in size.
Phillip
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