Re: [PATCH] mm/khugepaged: return -EAGAIN for SCAN_PAGE_HAS_PRIVATE in MADV_COLLAPSE

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 29 Apr 2026 19:34:34 +0530 Vineet Agarwal <agarwal.vineet2006@gmail.com> wrote:

> MADV_COLLAPSE uses errno values to provide actionable feedback to
> userspace. Temporary resource constraints are mapped to -EAGAIN so the
> caller may retry, while intrinsic failures of the specified range are
> mapped to -EINVAL.
> 
> collapse_file() returns SCAN_PAGE_HAS_PRIVATE when
> filemap_release_folio() fails while isolating file-backed folios for
> collapse. This currently falls through the default case in
> madvise_collapse_errno() and is reported to userspace as -EINVAL.
> 
> However, filemap_release_folio() failure commonly reflects temporary
> folio state rather than a permanently uncollapsible range.
> 
> For example, ext4 returns false when a folio still has dirty
> journalled data, btrfs returns false for dirty or writeback folios
> before extent state release, and NFS may return false while reclaiming
> filesystem-private folio state.
> 
> In such cases, retrying MADV_COLLAPSE after writeback, reclaim or
> journal progress may succeed. This matches the existing -EAGAIN
> handling for SCAN_PAGE_DIRTY_OR_WRITEBACK and other transient collapse
> failures more closely than -EINVAL.
> 
> Therefore, map SCAN_PAGE_HAS_PRIVATE to -EAGAIN so userspace receives
> retryable feedback for this temporary failure path.

Seems very reasonable to me.

The madvise(2) manpage could be a lot more helpful here.

	EAGAIN A kernel resource was temporarily unavailable.

and that's it!