Re: [PATCH v7 01/13] memory-failure: Fix hardware poison check in unpoison_memory() again

[Gregory Price <gourry@gourry.net>]

On Tue, Jul 28, 2026 at 09:43:53PM +0100, Matthew Wilcox (Oracle) wrote:
> The earlier patch (6c54312f9689) that attempted to fix unpoison_memory()
> was incorrect for hugetlb folios.  Before a6fddef49eef, we checked
> the head page for poison (which was correct for hugetlb and incorrect
> for THP).  We are currently incapable of unpoisoning pages (other than
> the first page) in a hugetlb folio.
> 
> Use is_page_hwpoison() which handles hugetlb pages sufficiently well for
> this purpose.  This is racy as we don't have a reference on the folio at
> this point, but fixing that properly requires deeper surgery and this
> is a CAP_SYS_ADMIN path only so this will do for the moment and can be
> easily backported.  It's no worse than the situation before a6fddef49eef.
> 
> The other bug in a6fddef49eef is that we currently clear the HWPoison
> flag on the precise page, which is wrong for hugetlb.  Fix that too.
> 
> Fixes: a6fddef49eef ("mm/memory-failure: convert unpoison_memory() to folios")
> Cc: stable@vger.kernel.org
> Cc: Miaohe Lin <linmiaohe@huawei.com>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

Just trying to wrap my head around the state before/after, assuming
the following is correct then LGTM:

prior:
   pfn_to_online_page(pfn) -> p = precise page
   PageHWPoison(page)      -> checks precise page. Poison for hugetlb
                              is only set only on the head, so bug.
after:
   is_page_hwpoison(page)  -> checks precise page AND head (if hugetlb)
   p = folio->page         -> now TestClearPageHWPoison() explicitly
                              clears the head page poison if hugetlb

Reviewed-by: Gregory Price (Meta) <gourry@gourry.net>