Date:   Wed, 13 Apr 2022 18:20:27 +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 14:48, Matthew Wilcox wrote:
> On Wed, Apr 13, 2022 at 02:28:38PM +0200, David Hildenbrand wrote:
>> 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.
> 
> I have the start of a series which starts looking at the fault path
> to see where it makes sense to use folios and where it makes sense to
> use pages.
> 
> We're (generally) faulting on a PTE, so we need the precise page to
> be returned in vmf->page.  However vmf->cow_page can/should be a
> folio (because it's definitely not a tail page).  That trickles
> down into copy_present_page() (new_page and prealloc both become folios)
> and so page_add_new_anon_rmap() then looks like a good target to
> take a folio.
> 
> The finish_fault() -> do_set_pte() -> page_add_new_anon_rmap() looks
> like the only kind of strange place where we don't necessarily have a
> folio (all the others we just allocated it).
> 

That's an interesting point. In this patch I'm assuming that we don't
have a compound page here (see below).

Which makes sense, because as the interface states "Same as
page_add_anon_rmap but must only be called on *new* pages.".

At least to me it would be weird to allocate a new compound page to then
pass a subpage to do_set_pte() page_add_new_anon_rmap().


And in fact, inside page_add_new_anon_rmap(compound=false) we have

/* Anon THP always mapped first with PMD */
VM_BUG_ON_PAGE(PageTransCompound(page), page);


which makes sure that we cannot have a compound page here, but in fact a
folio.

So unless I am missing something, do_set_pte() should in fact have a
folio here unless BUG?

-- 
Thanks,

David / dhildenb

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