Re: [PATCH 7.2 v2 06/12] mm: fs: remove filemap_nr_thps*() functions and their users

[Matthew Wilcox <willy@infradead.org>]

On Mon, Apr 13, 2026 at 03:20:24PM -0400, Zi Yan wrote:
> They are used by READ_ONLY_THP_FOR_FS to handle writes to FSes without
> large folio support, so that read-only THPs created in these FSes are not
> seen by the FSes when the underlying fd becomes writable. Now read-only PMD
> THPs only appear in a FS with large folio support and the supported orders
> include PMD_ORDRE.
> 
> READ_ONLY_THP_FOR_FS was using mapping->nr_thps, inode->i_writecount, and
> smp_mb() to prevent writes to a read-only THP and collapsing writable
> folios into a THP. In collapse_file(), mapping->nr_thps is increased, then
> smp_mb(), and if inode->i_writecount > 0, collapse is stopped, while
> do_dentry_open() first increases inode->i_writecount, then a full memory
> fence, and if mapping->nr_thps > 0, all read-only THPs are truncated.
> 
> Now this mechanism can be removed along with READ_ONLY_THP_FOR_FS code,
> since a dirty folio check has been added after try_to_unmap() and
> try_to_unmap_flush() in collapse_file() to make sure no writable folio can
> be collapsed.
> 
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> ---
>  fs/open.c               | 27 ---------------------------
>  include/linux/pagemap.h | 29 -----------------------------
>  mm/filemap.c            |  1 -
>  mm/huge_memory.c        |  1 -
>  mm/khugepaged.c         | 28 ----------------------------
>  5 files changed, 86 deletions(-)

This is great.

Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>