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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sat, 19 Dec 2020 12:08:21 -0800
Message-ID: <CAHk-=wgwf_JUS+0kQas8SWLs3bkpF55xCoWPOACFsdgwonuZ8g@mail.gmail.com>
Subject: Re: [PATCH 1/2] mm: Allow architectures to request 'old' entries when prefaulting
To:     "Kirill A. Shutemov" <kirill@shutemov.name>
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>
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On Sat, Dec 19, 2020 at 4:41 AM Kirill A. Shutemov <kirill@shutemov.name> wrote:
>
> Okay, but we only win the NULL check. xas_retry() and xa_is_value() has to
> be repeated in the beginning of the loop.

Yeah.

I wonder if it might make sense to have a "xas_next_entry_rcu()"
function that does something like

    while ((entry = xas_next_entry(&xas, end)) != NULL) {
            if (xas_retry(entry) || xa_is_value(entry))
                    continue;
            return entry;
    }
    return NULL;

but that's a question for Willy, and independent of this patch.

I like the patch, and I'll think about it a bit more, but I might
decide to just apply it to get this thing over with.

Otherwise it should probably go into -mm for more testing.

                Linus