linux-kernel - Re: [PATCH v3 04/16] mm/rmap: split page_dup_rmap() into page_dup_file_rmap() and page_try_dup_anon

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Message-ID: <7d9353f5-2aa4-1e70-deb2-2ff3d2afb396@redhat.com>
Date:   Tue, 12 Apr 2022 10:06:03 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Vlastimil Babka <vbabka@...e.cz>, 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 04/16] mm/rmap: split page_dup_rmap() into
 page_dup_file_rmap() and page_try_dup_anon_rmap()

On 11.04.22 20:18, Vlastimil Babka wrote:
> On 3/29/22 18:04, David Hildenbrand wrote:
>> ... and move the special check for pinned pages into
>> page_try_dup_anon_rmap() to prepare for tracking exclusive anonymous
>> pages via a new pageflag, clearing it only after making sure that there
>> are no GUP pins on the anonymous page.
>>
>> We really only care about pins on anonymous pages, because they are
>> prone to getting replaced in the COW handler once mapped R/O. For !anon
>> pages in cow-mappings (!VM_SHARED && VM_MAYWRITE) we shouldn't really
>> care about that, at least not that I could come up with an example.
>>
>> Let's drop the is_cow_mapping() check from page_needs_cow_for_dma(), as we
>> know we're dealing with anonymous pages. Also, drop the handling of
>> pinned pages from copy_huge_pud() and add a comment if ever supporting
>> anonymous pages on the PUD level.
>>
>> This is a preparation for tracking exclusivity of anonymous pages in
>> the rmap code, and disallowing marking a page shared (-> failing to
>> duplicate) if there are GUP pins on a page.
>>
>> Signed-off-by: David Hildenbrand <david@...hat.com>
> 
> Acked-by: Vlastimil Babka <vbabka@...e.cz>
> 
> Nit:
> 
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -825,7 +825,8 @@ copy_nonpresent_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
>>  		 */
>>  		get_page(page);
>>  		rss[mm_counter(page)]++;
>> -		page_dup_rmap(page, false);
>> +		/* Cannot fail as these pages cannot get pinned. */
>> +		BUG_ON(page_try_dup_anon_rmap(page, false, src_vma));
> 
> Should we just call __page_dup_rmap() here? This is block for the condition
> is_device_private_entry(), and page_try_dup_anon_rmap() can't return -EBUSY
> for is_device_private_page().


Hi Vlastimil,

thanks for your review!

We want to keep page_try_dup_anon_rmap() here, because we extend
page_try_dup_anon_rmap() in patch #12 to properly clear
PageAnonExclusive() of there are no GUP pins. Just like with current
page_try_dup_anon_rmap(), that can't fail for device private pages.

-- 
Thanks,

David / dhildenb