Re: [PATCH v3 04/16] mm/rmap: split page_dup_rmap() into page_dup_file_rmap() and page_try_dup_anon_rmap()

From: David Hildenbrand <david@redhat.com>
To: Vlastimil Babka <vbabka@suse.cz>, linux-kernel@vger.kernel.org
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Hugh Dickins <hughd@google.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	David Rientjes <rientjes@google.com>,
	Shakeel Butt <shakeelb@google.com>,
	John Hubbard <jhubbard@nvidia.com>,
	Jason Gunthorpe <jgg@nvidia.com>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Mike Rapoport <rppt@linux.ibm.com>,
	Yang Shi <shy828301@gmail.com>,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
	Matthew Wilcox <willy@infradead.org>,
	Jann Horn <jannh@google.com>, Michal Hocko <mhocko@kernel.org>,
	Nadav Amit <namit@vmware.com>, Rik van Riel <riel@surriel.com>,
	Roman Gushchin <guro@fb.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Peter Xu <peterx@redhat.com>, Donald Dutile <ddutile@redhat.com>,
	Christoph Hellwig <hch@lst.de>, Oleg Nesterov <oleg@redhat.com>,
	Jan Kara <jack@suse.cz>, Liang Zhang <zhangliang5@huawei.com>,
	Pedro Gomes <pedrodemargomes@gmail.com>,
	Oded Gabbay <oded.gabbay@gmail.com>,
	linux-mm@kvack.org
Subject: Re: [PATCH v3 04/16] mm/rmap: split page_dup_rmap() into page_dup_file_rmap() and page_try_dup_anon_rmap()
Date: Tue, 12 Apr 2022 10:06:03 +0200	[thread overview]
Message-ID: <7d9353f5-2aa4-1e70-deb2-2ff3d2afb396@redhat.com> (raw)
In-Reply-To: <bec1b401-fb2e-6672-d904-396402434890@suse.cz>

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@redhat.com>
> 
> Acked-by: Vlastimil Babka <vbabka@suse.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