linux-kernel - Re: [PATCH v3 08/16] mm/rmap: drop "compound" parameter from page_add_new_anon

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Date:   Wed, 13 Apr 2022 14:28:38 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org,
        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>,
        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 08/16] mm/rmap: drop "compound" parameter from
 page_add_new_anon_rmap()

On 13.04.22 14:26, Matthew Wilcox wrote:
> On Tue, Apr 12, 2022 at 11:37:09AM +0200, David Hildenbrand wrote:
>> On 12.04.22 10:47, Vlastimil Babka wrote:
>>> There's a VM_BUG_ON_PAGE(PageTransCompound(page), page); later in a
>>> !compound branch. Since compound is now determined by the same check, could
>>> be deleted.
>>
>> Yes, eventually we could get rid of both VM_BUG_ON_PAGE() on both
>> branches and add a single VM_BUG_ON_PAGE(PageTail(page), page) check on
>> the compound branch. (we could also make sure that we're not given a
>> hugetlb page)
> 
> As a rule of thumb, if you find yourself wanting to add
> VM_BUG_ON_PAGE(PageTail(page), page), you probably want to change the
> interface to take a folio.

Yeah, I had the same in mind. Might be a reasonable addon on top --
although it would stick out in the rmap code a bit because most
functions deal with both, folios and subpages.

-- 
Thanks,

David / dhildenb