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Date:   Thu, 17 Dec 2020 13:54:09 +0300
From:   "Kirill A. Shutemov" <kirill@shutemov.name>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Matthew Wilcox <willy@infradead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Will Deacon <will@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Jan Kara <jack@suse.cz>, Minchan Kim <minchan@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vinayak Menon <vinmenon@codeaurora.org>,
        Android Kernel Team <kernel-team@android.com>
Subject: Re: [PATCH 1/2] mm: Allow architectures to request 'old' entries
 when prefaulting
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On Wed, Dec 16, 2020 at 10:41:36AM -0800, Linus Torvalds wrote:
> On Wed, Dec 16, 2020 at 9:07 AM Kirill A. Shutemov <kirill@shutemov.name> wrote:
> >
> > If this looks fine, I'll submit a proper patch.
> 
> That patch looks good to me.
> 
> It would be good if somebody else looked it through - maybe I like it
> just because I got to pee in the snow and make my mark. But i think
> that filemap_map_pages() now looks a lot more understandable, and
> having that pte_offset_map_lock() outside the loop should be good.

It worth noting that after the change in the worth case scenario we will
have additional ref/unref and lock/unlock of the page if we get deep
enough into filemap_map_pmd(), but fail to map the page.

Also if the range doesn't have a mappable page we would setup a page
table into the PMD entry. It means we cannot have huge page mapped there
later. It may be a bummer: getting the page table out of page table tree
requires mmap_write_lock().

We also take ptl for cold page cache. It may increase ptl contention, but
it should be negligible with split-ptl.

-- 
 Kirill A. Shutemov