Re: [RESEND PATCH v2] btrfs: prevent direct reclaim during compressed readahead

[David Sterba]

On Sat, Mar 28, 2026 at 02:46:19PM -0700, JP Kobryn (Meta) wrote:
> 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@xxxxxx/
> 
> Signed-off-by: JP Kobryn (Meta) <jp.kobryn@xxxxxxxxx>
> Reviewed-by: Mark Harmstone <mark@xxxxxxxxxxxxx>
> ---
> v2:
>  - dropped patch 1/2, squashed into single patch based on David's feedback
>  - changed btrfs_alloc_compr_folio() signature instead of new _gfp variant
>  - update other existing callers to pass GFP_NOFS explicitly

Added to for-next, thanks.