[26.04-linux-nvidia] Backport: fuse: back uncached readdir buffers with pages#459
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[26.04-linux-nvidia] Backport: fuse: back uncached readdir buffers with pages#459nvmochs wants to merge 1 commit into
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Commit dabb903 ("fuse: increase readdir buffer size") changed fuse_readdir_uncached() to size its temporary buffer from ctx->count. This is useful for overlayfs and other in-kernel callers that use INT_MAX to indicate an unlimited directory read. The larger buffer is currently supplied as a kvec output argument. For virtiofs, kvec arguments are copied through req->argbuf, which is allocated with kmalloc(..., GFP_ATOMIC). A large uncached readdir buffer can therefore require a multi-megabyte contiguous atomic allocation before the request is queued. Avoid the large bounce-buffer allocation by backing uncached readdir output with pages and setting out_pages. Transports such as virtiofs can then pass the pages as scatter-gather entries instead of copying the output through argbuf. Map the pages with vm_map_ram() only while parsing the returned dirents. The existing parser can then continue to use a linear kernel mapping. [SzM: separate allocation of pages into a helper function] Fixes: dabb903 ("fuse: increase readdir buffer size") Cc: stable@vger.kernel.org Signed-off-by: Matthew R. Ochs <mochs@nvidia.com> Signed-off-by: Miklos Szeredi <mszeredi@redhat.com> (cherry picked from commit 2fcb1dd15faba5657b825cc5d54423251c70e79a linux-next) Signed-off-by: Matthew R. Ochs <mochs@nvidia.com>
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PR Validation ReportPatchscan ✅ No Missing FixesAll cherry-picked commits checked — no missing upstream fixes found. PR Lint ❌ Errors foundDetailsChecking 1 commits...
Cherry-pick digest:
┌──────────────┬──────────────────────────────────────────────────────────────────┬────────────┬─────────┬───────────────────────────┐
│ Local │ Referenced upstream / Patch subject │ Patch-ID │ Subject │ SoB chain │
├──────────────┼──────────────────────────────────────────────────────────────────┼────────────┼─────────┼───────────────────────────┤
│ 3d12b88b9368 │ [SAUCE] fuse: back uncached readdir buffers with pages │ N/A │ N/A │ mochs, mszeredi, mochs │
└──────────────┴──────────────────────────────────────────────────────────────────┴────────────┴─────────┴───────────────────────────┘
Lint results:
E: 3d12b88b9368 ("fuse: back uncached readdir buffers with pages"): not SAUCE/UBUNTU/Revert but has no upstream reference trailer (cherry picked from commit ... or backported from ...)
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Acked-by: Carol L Soto <csoto@nvidia.com>
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BaseOS Kernel ReviewSummaryIn fuse readdir (fs/fuse/readdir.c:396), switching uncached readdir buffers to vm_map_ram()/vm_unmap_ram() will panic on NOMMU systems where those calls are BUG(), unlike the prior kvmalloc() path; since FUSE_FS has no MMU dependency, this is a NOMMU regression. Findings: Critical: 1, High: 0, Medium: 0, Low: 0 Latest watcher review: open review Kernel deb build: successful (download debs, 4 files) Head: This comment is maintained by nv-pr-bot. It is updated when the GitHub watcher publishes a newer review. |
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Merged, closing PR. |
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This updates FUSE uncached readdir to back the temporary output buffer with pages and set out_pages, instead of passing the buffer as a kvec output argument.
For virtiofs, kvec output arguments are copied through req->argbuf, which is allocated with kmalloc(..., GFP_ATOMIC). Large uncached readdir requests can therefore require a multi-megabyte contiguous atomic allocation before the request is queued. Using output pages lets transports such as virtiofs pass the buffer as scatter-gather entries instead.
The existing uncached readdir buffer sizing is unchanged. Pages are mapped with vm_map_ram() only while parsing returned dirents.
LKML:
[PATCH v4] fuse: back uncached readdir buffers with pages - https://lore.kernel.org/all/20260526152021.891412-1-mochs@nvidia.com/
linux-next:
2fcb1dd15fab fuse: back uncached readdir buffers with pages
Testing
Validated the original failing configuration:
Command:
Result:
This is the configuration that previously failed with Cannot allocate memory.
LP: https://bugs.launchpad.net/ubuntu/+source/linux-nvidia/+bug/2156632
NVbug: 6059208