Re: [f2fs-dev] [PATCH 05/14] f2fs: support large folio writeback

[Nanzhe Zhao]

On Thu, 3 Sep 2026 21:04:36 -0700, Daeho Jeong wrote:
> In __has_merged_page(), target->mapping is directly dereferenced. For tail
> pages, target->mapping contains compound_head metadata rather than a valid
> address_space pointer, leading to a kernel crash when accessing
> target->mapping->host.

Thanks for the review.

I've been testing on 6.18-based (Android) and 7.2-rc5 trees, and in
both of them bio_for_each_folio_all() goes through bio_first_folio(),
which resolves the head folio from the bvec directly:

    fi->folio = page_folio(bvec->bv_page);

So the folio yielded to __has_merged_page() should always be the head
folio there.

Are you testing on a 6.12-based Android kernel? In 6.12,
__has_merged_page() still iterated with bio_for_each_segment_all() and
took bvec->bv_page directly as target, so a tail page would make
target->mapping point at the compound_head metadata and crash on
target->mapping->host.

If so, would it be possible to retest on a 6.18 Android kernel, where
the iterator normalizes to the head folio?

Thanks,
Nanzhe