kernel/linux.git/fs/btrfs/compression.h, branch linux-7.1.y

btrfs: prevent direct reclaim during compressed readahead

2026-04-07T16:56:08+00:00

Under memory pressure, direct reclaim can kick in during compressed readahead. This puts the associated task into D-state. Then shrink_lruvec() disables interrupts when acquiring the LRU lock. Under heavy pressure, we've observed reclaim can run long enough that the CPU becomes prone to CSD lock stalls since it cannot service incoming IPIs. Although the CSD lock stalls are the worst case scenario, we have found many more subtle occurrences of this latency on the order of seconds, over a minute in some cases. Prevent direct reclaim during compressed readahead. This is achieved by using different GFP flags at key points when the bio is marked for readahead. There are two functions that allocate during compressed readahead: btrfs_alloc_compr_folio() and add_ra_bio_pages(). Both currently use GFP_NOFS which includes __GFP_DIRECT_RECLAIM. For the internal API call btrfs_alloc_compr_folio(), the signature changes to accept an additional gfp_t parameter. At the readahead call site, it gets flags similar to GFP_NOFS but stripped of __GFP_DIRECT_RECLAIM. __GFP_NOWARN is added since these allocations are allowed to fail. Demand reads still use full GFP_NOFS and will enter reclaim if needed. All other existing call sites of btrfs_alloc_compr_folio() now explicitly pass GFP_NOFS to retain their current behavior. add_ra_bio_pages() gains a bool parameter which allows callers to specify if they want to allow direct reclaim or not. In either case, the __GFP_NOWARN flag was added unconditionally since the allocations are speculative. There has been some previous work done on calling add_ra_bio_pages() [0]. This patch is complementary: where that patch reduces call frequency, this patch reduces the latency associated with those calls. [0] https://lore.kernel.org/linux-btrfs/656838ec1232314a2657716e59f4f15a8eadba64.1751492111.git.boris@bur.io/ Reviewed-by: Mark Harmstone Reviewed-by: Qu Wenruo Signed-off-by: JP Kobryn (Meta) Reviewed-by: David Sterba Signed-off-by: David Sterba

btrfs: extract the max compression chunk size into a macro

2026-04-07T16:56:00+00:00

We have two locations using open-coded 512K size, as the async chunk size. For compression we have not only the max size a compressed extent can represent (128K), but also how large an async chunk can be (512K). Although we have a macro for the maximum compressed extent size, we do not have any macro for the async chunk size. Add such a macro and replace the two open-coded SZ_512K. Reviewed-by: Filipe Manana Signed-off-by: Qu Wenruo Signed-off-by: David Sterba

btrfs: reduce the size of compressed_bio

2026-04-07T16:55:59+00:00

The member compressed_bio::compressed_len can be replaced by the bio size, as we always submit the full compressed data without any partial read/write. Furthermore we already have enough ASSERT()s making sure the bio size matches the ordered extent or the extent map. This saves 8 bytes from compressed_bio: Before: struct compressed_bio { u64 start; /* 0 8 */ unsigned int len; /* 8 4 */ unsigned int compressed_len; /* 12 4 */ u8 compress_type; /* 16 1 */ bool writeback; /* 17 1 */ /* XXX 6 bytes hole, try to pack */ struct btrfs_bio * orig_bbio; /* 24 8 */ struct btrfs_bio bbio __attribute__((__aligned__(8))); /* 32 304 */ /* XXX last struct has 1 bit hole */ /* size: 336, cachelines: 6, members: 7 */ /* sum members: 330, holes: 1, sum holes: 6 */ /* member types with bit holes: 1, total: 1 */ /* forced alignments: 1 */ /* last cacheline: 16 bytes */ } __attribute__((__aligned__(8))); After: struct compressed_bio { u64 start; /* 0 8 */ unsigned int len; /* 8 4 */ u8 compress_type; /* 12 1 */ bool writeback; /* 13 1 */ /* XXX 2 bytes hole, try to pack */ struct btrfs_bio * orig_bbio; /* 16 8 */ struct btrfs_bio bbio __attribute__((__aligned__(8))); /* 24 304 */ /* XXX last struct has 1 bit hole */ /* size: 328, cachelines: 6, members: 6 */ /* sum members: 326, holes: 1, sum holes: 2 */ /* member types with bit holes: 1, total: 1 */ /* forced alignments: 1 */ /* last cacheline: 8 bytes */ } __attribute__((__aligned__(8))); Signed-off-by: Qu Wenruo Reviewed-by: David Sterba Signed-off-by: David Sterba

btrfs: get rid of compressed_bio::compressed_folios[]

2026-02-03T06:59:07+00:00

Now there is no one utilizing that member, we can safely remove it along with compressed_bio::nr_folios member. The size is reduced from 352 to 336 bytes on x86_64. Reviewed-by: Boris Burkov Signed-off-by: Qu Wenruo Reviewed-by: David Sterba Signed-off-by: David Sterba

btrfs: get rid of compressed_folios[] usage for encoded writes

2026-02-03T06:59:07+00:00

Currently only encoded writes utilized btrfs_submit_compressed_write(), which utilized compressed_bio::compressed_folios[] array. Change the only call site to call the new helper, btrfs_alloc_compressed_write(), to allocate a compressed bio, then queue needed folios into that bio, and finally call btrfs_submit_compressed_write() to submit the compressed bio. This change has one hidden benefit, previously we used btrfs_alloc_folio_array() for the folios of btrfs_submit_compressed_read(), which doesn't utilize the compression page pool for bs == ps cases. Now we call btrfs_alloc_compr_folio() which will benefit from the page pool. The other obvious benefit is that we no longer need to allocate an array to hold all those folios, thus one less error path. Reviewed-by: Boris Burkov Signed-off-by: Qu Wenruo Reviewed-by: David Sterba Signed-off-by: David Sterba

btrfs: remove the old btrfs_compress_folios() infrastructure

2026-02-03T06:59:07+00:00

Since it's been replaced by btrfs_compress_bio(), remove all involved functions. Reviewed-by: Boris Burkov Signed-off-by: Qu Wenruo Reviewed-by: David Sterba Signed-off-by: David Sterba

btrfs: introduce btrfs_compress_bio() helper

2026-02-03T06:59:06+00:00

The helper will allocate a new compressed_bio, do the compression, and return it to the caller. This greatly simplifies the compression path, as we no longer need to allocate a folio array thus no extra error path, furthermore the compressed bio structure can be utilized for submission with very minor modifications (like rounding up the bi_size and populate the bi_sector). Reviewed-by: Boris Burkov Signed-off-by: Qu Wenruo Reviewed-by: David Sterba Signed-off-by: David Sterba

btrfs: zlib: introduce zlib_compress_bio() helper

2026-02-03T06:59:06+00:00

The new helper has the following enhancements against the existing zlib_compress_folios() - Much smaller parameter list No more shared IN/OUT members, no need to pre-allocate a compressed_folios[] array. Just a workspace and compressed_bio pointer, everything we need can be extracted from that @cb pointer. - Ready-to-be-submitted compressed bio Although the caller still needs to do some common works like rounding up and zeroing the tailing part of the last fs block. Reviewed-by: Boris Burkov Signed-off-by: Qu Wenruo Reviewed-by: David Sterba Signed-off-by: David Sterba

btrfs: zstd: introduce zstd_compress_bio() helper

2026-02-03T06:59:06+00:00

The new helper has the following enhancements against the existing zstd_compress_folios() - Much smaller parameter list No more shared IN/OUT members, no need to pre-allocate a compressed_folios[] array. Just a workspace and compressed_bio pointer, everything we need can be extracted from that @cb pointer. - Ready-to-be-submitted compressed bio Although the caller still needs to do some common works like rounding up and zeroing the tailing part of the last fs block. Overall the workflow is the same as zstd_compress_folios(), but with some minor changes: - @start/@len is now constant For the current input file offset, use @start + @tot_in instead. The original change of @start and @len makes it pretty hard to know what value we're really comparing to. - No more @cur_len It's only utilized when switching input buffer. Directly use btrfs_calc_input_length() instead. Reviewed-by: Boris Burkov Signed-off-by: Qu Wenruo Reviewed-by: David Sterba Signed-off-by: David Sterba

btrfs: lzo: introduce lzo_compress_bio() helper

2026-02-03T06:59:06+00:00

The new helper has the following enhancements against the existing lzo_compress_folios() - Much smaller parameter list No more shared IN/OUT members, no need to pre-allocate a compressed_folios[] array. Just a workspace list header and a compressed_bio pointer. Everything else can be fetched from that @cb pointer. - Read-to-be-submitted compressed bio Although the caller still needs to do some common works like rounding up and zeroing the tailing part of the last fs block. Some workloads are specific to lZO that is not needed with other multi-run compression interfaces: - Need to write a LZO header or segment header Use the new write_and_queue_folio() helper to do the bio_add_folio() call and folio switching. - Need to update the LZO header after compression is done Use bio_first_folio_all() to grab the first folio and update the header. - Extra corner case of error handling This can happen when we have queued part of a folio and hit an error. In that case those folios will be released by the bio. Thus we can only release the folio that has no queued part. Reviewed-by: Boris Burkov Signed-off-by: Qu Wenruo Reviewed-by: David Sterba Signed-off-by: David Sterba