Re: [PATCH] fs/proc/task_mmu: check THP exclusivity for each subpage

[Andrew Morton]

On Sun,  1 Nov 2026 09:01:00 -0800 Kelvin Zhang <zhangxp1998@xxxxxxxxx> wrote:

> A PMD-mapped THP can contain subpages with different mapcounts. For
> example, after fork(), the child can unmap only part of the THP. The
> parent retains its PMD mapping, while the remaining child mappings
> are PTE mappings.
> 
> pagemap_pmd_range_thp() passes the same initial page to
> __folio_page_mapped_exclusively() for every pagemap entry. With
> CONFIG_PAGE_MAPCOUNT=y, this applies the first subpage's exclusivity
> to all entries, falsely setting or clearing PM_MMAP_EXCLUSIVE for
> subpages with different sharing.
> 
> Pass page + idx to check the subpage corresponding to each entry, as
> was done before converting to __folio_page_mapped_exclusively(). The
> index already accounts for reads starting partway through the PMD.
> The conservative whole-folio check without CONFIG_PAGE_MAPCOUNT is
> unchanged.

Thanks.

I don't want to pick on you at all, but this is a classic changelog - I
see this pattern so often.

	a) here's what the code does

	b) here's why that's wrong

	c) here's how I fixed it.

But that's missing a very important bullet point:

	a) here's what the code does

	b) here's why that's wrong

	c) here's how this affects users

	d) here's how I fixed it.

see?  This omission is so common!  And I might have sent one or two
emails asking people for the missing info.

For the missing c), how does this sound?

: With transparent huge pages, /proc/PID/pagemap can incorrectly
: report shared pages as exclusive, or vice versa, after fork() and
: partial unmapping.  This can mislead memory-analysis and
: checkpoint/restore tools.  Actual mappings and copy-on-write behavior
: are unaffected.

And now that we have our statement of userspace impact, busy non-mm
downstream readers of this patch can easily see why your proposed
cc:stable is justified.


Also, it would be helpful to show brief before-and-after excerpts from
/proc/PID/pagemap to help others understand the impact of the change. 
But I don't think we need a v2 just for this.