linux-kernel - Re: [PATCH v3 10/16] mm/huge_memory: remove outdated VM_WARN_ON_ONCE_PAGE from unmap

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Date:   Tue, 12 Apr 2022 11:37:08 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Yang Shi <shy828301@...il.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Jann Horn <jannh@...gle.com>, Michal Hocko <mhocko@...nel.org>,
        Nadav Amit <namit@...are.com>, Rik van Riel <riel@...riel.com>,
        Roman Gushchin <guro@...com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        Donald Dutile <ddutile@...hat.com>,
        Christoph Hellwig <hch@....de>,
        Oleg Nesterov <oleg@...hat.com>, Jan Kara <jack@...e.cz>,
        Liang Zhang <zhangliang5@...wei.com>,
        Pedro Gomes <pedrodemargomes@...il.com>,
        Oded Gabbay <oded.gabbay@...il.com>, linux-mm@...ck.org
Subject: Re: [PATCH v3 10/16] mm/huge_memory: remove outdated
 VM_WARN_ON_ONCE_PAGE from unmap_page()

On 3/29/22 18:04, David Hildenbrand wrote:
> We can already theoretically fail to unmap (still having page_mapped()) in
> case arch_unmap_one() fails, which can happen on sparc. Failures to
> unmap are handled gracefully, just as if there are other references on
> the target page: freezing the refcount in split_huge_page_to_list()
> will fail if still mapped and we'll simply remap.
> 
> In commit 504e070dc08f ("mm: thp: replace DEBUG_VM BUG with VM_WARN when
> unmap fails for split") we already converted to VM_WARN_ON_ONCE_PAGE,
> let's get rid of it completely now.
> 
> This is a preparation for making try_to_migrate() fail on anonymous pages
> with GUP pins, which will make this VM_WARN_ON_ONCE_PAGE trigger more
> frequently.
> 
> Reported-by: Yang Shi <shy828301@...il.com>
> Reviewed-by: Yang Shi <shy828301@...il.com>
> Signed-off-by: David Hildenbrand <david@...hat.com>

Acked-by: Vlastimil Babka <vbabka@...e.cz>

> ---
>  mm/huge_memory.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index dd16819c5edc..70298431e128 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2241,8 +2241,6 @@ static void unmap_page(struct page *page)
>  		try_to_migrate(folio, ttu_flags);
>  	else
>  		try_to_unmap(folio, ttu_flags | TTU_IGNORE_MLOCK);
> -
> -	VM_WARN_ON_ONCE_PAGE(page_mapped(page), page);
>  }
>  
>  static void remap_page(struct folio *folio, unsigned long nr)