Re: [PATCH] btrfs: always pass __GFP_NOWARN from add_ra_bio_pages()

[David Sterba]

On Wed, Apr 29, 2026 at 12:10:25AM -0700, Calvin Owens wrote:
> A build workload newly prints order-0 allocation failures on 7.1-rc1:
> 
>     sh: page allocation failure: order:0
>     mode:0x14084a(__GFP_HIGHMEM|__GFP_MOVABLE|__GFP_IO|__GFP_KSWAPD_RECLAIM|
>                   __GFP_COMP|__GFP_HARDWALL)
>     CPU: 27 UID: 1000 PID: 855540 Comm: sh Not tainted 7.1.0-rc1-llvm-00058-gdca922e019dd #1 PREEMPTLAZY
>     Call Trace:
>      <TASK>
>      dump_stack_lvl+0x50/0x70
>      warn_alloc+0xeb/0x100
>      __alloc_pages_slowpath+0x567/0x5a0
>      ? filemap_get_entry+0x11a/0x140
>      __alloc_frozen_pages_noprof+0x249/0x2d0
>      alloc_pages_mpol+0xe4/0x180
>      folio_alloc_noprof+0x80/0xa0
>      add_ra_bio_pages+0x13c/0x4b0
>      btrfs_submit_compressed_read+0x229/0x300
>      submit_one_bio+0x9e/0xe0
>      btrfs_readahead+0x185/0x1a0
>      [...]
> 
>     (lldb) source list -a add_ra_bio_pages+0x13c
>     .../vmlinux.unstripped add_ra_bio_pages + 316 at .../fs/btrfs/compression.c:454:8
>        451
>        452                  folio = filemap_alloc_folio(mapping_gfp_constraint(mapping, constraint_gfp),
>        453                                              0, NULL);
>     -> 454                  if (!folio)
>        455                          break;
> 
> I can reproduce this consistently by running a memory hog concurrently
> with a buffered writer on a machine with a very large amount of swap.
> 
> Commit 7ae37b2c94ed ("btrfs: prevent direct reclaim during compressed
> readahead") clearly intended to suppress these warnings. But because the
> mask set in the address_space with mapping_set_gfp_mask() doesn't include
> __GFP_NOWARN, mapping_gfp_constraint() removes it from constraint_gfp
> before it is passed to filemap_alloc_folio().
> 
> Fix by refactoring the code to add __GFP_NOWARN after the call to
> mapping_gfp_constraint().
> 
> Fixes: 7ae37b2c94ed ("btrfs: prevent direct reclaim during compressed readahead")
> Signed-off-by: Calvin Owens <calvin@xxxxxxxxxx>

This is on me, the original patch https://lore.kernel.org/linux-btrfs/20260323051414.64704-1-jp.kobryn@xxxxxxxxx/
had the __GFP_NOWARN at the allocation. As this was not explained as
necessary I moved it to the constant assignments to be grouped. But this
changed the semantics.

Added to for-next, thanks.