linux-kernel - Re: [PATCH 1/2] mm: Allow architectures to request 'old' entries when prefaulting

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Date:   Thu, 17 Dec 2020 13:54:09 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Will Deacon <will@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Jan Kara <jack@...e.cz>, Minchan Kim <minchan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vinayak Menon <vinmenon@...eaurora.org>,
        Android Kernel Team <kernel-team@...roid.com>
Subject: Re: [PATCH 1/2] mm: Allow architectures to request 'old' entries
 when prefaulting

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@...temov.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