linux-kernel - Re: [PATCH v2 05/32] mm/filemap: allow pte_offset_map

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Date:   Mon, 10 Jul 2023 21:34:36 -0400
From:   Zi Yan <ziy@...dia.com>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Qi Zheng <zhengqi.arch@...edance.com>,
        Yang Shi <shy828301@...il.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Peter Xu <peterx@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
        Alistair Popple <apopple@...dia.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Steven Price <steven.price@....com>,
        SeongJae Park <sj@...nel.org>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        Huang Ying <ying.huang@...el.com>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Zack Rusin <zackr@...are.com>, Jason Gunthorpe <jgg@...pe.ca>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Minchan Kim <minchan@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        Song Liu <song@...nel.org>,
        Thomas Hellstrom <thomas.hellstrom@...ux.intel.com>,
        Ryan Roberts <ryan.roberts@....com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2 05/32] mm/filemap: allow pte_offset_map_lock() to fail

On 8 Jun 2023, at 21:11, Hugh Dickins wrote:

> filemap_map_pages() allow pte_offset_map_lock() to fail; and remove the
> pmd_devmap_trans_unstable() check from filemap_map_pmd(), which can safely
> return to filemap_map_pages() and let pte_offset_map_lock() discover that.
>
> Signed-off-by: Hugh Dickins <hughd@...gle.com>
> ---
>  mm/filemap.c | 12 +++++-------
>  1 file changed, 5 insertions(+), 7 deletions(-)
>
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 28b42ee848a4..9e129ad43e0d 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -3408,13 +3408,6 @@ static bool filemap_map_pmd(struct vm_fault *vmf, struct folio *folio,
>  	if (pmd_none(*vmf->pmd))
>  		pmd_install(mm, vmf->pmd, &vmf->prealloc_pte);
>
> -	/* See comment in handle_pte_fault() */
> -	if (pmd_devmap_trans_unstable(vmf->pmd)) {
> -		folio_unlock(folio);
> -		folio_put(folio);
> -		return true;
> -	}
> -

There is a pmd_trans_huge() check at the beginning, should it be removed
as well? Since pte_offset_map_lock() is also able to detect it.

>  	return false;
>  }
>
> @@ -3501,6 +3494,11 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf,
>
>  	addr = vma->vm_start + ((start_pgoff - vma->vm_pgoff) << PAGE_SHIFT);
>  	vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, addr, &vmf->ptl);
> +	if (!vmf->pte) {
> +		folio_unlock(folio);
> +		folio_put(folio);
> +		goto out;
> +	}
>  	do {
>  again:
>  		page = folio_file_page(folio, xas.xa_index);
> -- 
> 2.35.3

These two changes affect the ret value. Before, pmd_devmap_trans_unstable() == true
made ret = VM_FAULT_NPAGE, but now ret is the default 0 value. So ret should be set
to VM_FAULT_NPAGE before goto out in the second hunk?

--
Best Regards,
Yan, Zi

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