linux-kernel - Re: [RFC PATCH] mm: khugepaged: don't carry huge page to the next loop for !CONFIG

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Date:   Wed, 22 Sep 2021 16:49:17 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Yang Shi <shy828301@...il.com>
cc:     Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Hugh Dickins <hughd@...gle.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] mm: khugepaged: don't carry huge page to the next
 loop for !CONFIG_NUMA

On Wed, 1 Sep 2021, Yang Shi wrote:
> On Wed, Sep 1, 2021 at 3:26 AM Vlastimil Babka <vbabka@...e.cz> wrote:
> > On 9/1/21 05:46, Yang Shi wrote:
> > > On Tue, Aug 31, 2021 at 4:38 PM Kirill A. Shutemov <kirill@...temov.name> wrote:
> > >> On Mon, Aug 30, 2021 at 11:49:43AM -0700, Yang Shi wrote:
> > >> > Gently ping...
> > >> >
> > >> > Does this patch make sense? BTW, I have a couple of other khugepaged
> > >> > related patches in my queue. I plan to send them with this patch
> > >> > together. It would be great to hear some feedback before resending
> > >> > this one.
> > >>
> > >> I don't really care for !NUMA optimization. I believe that most of setups
> > >> that benefit from THP has NUMA enabled compile time.
> > >
> > > Agreed.
> > >
> > >>
> > >> But if you wanna to go this path, make an effort to cleanup other
> > >> artifacts for the !NUMA optimization: the ifdef has to be gone and all
> > >> callers of these helpers has to be revisited. There's more opportunities to
> > >> cleanup. Like it is very odd that khugepaged_prealloc_page() frees the
> > >> page.
> > >
> > > Yes, they are gone in this patch. The only remaining for !NUMA is
> > > khugepaged_find_target_node() which just returns 0.
> >
> > As Kirill pointed out, there's also khugepaged_prealloc_page() where the
> > only remaining variant does actually no preallocation, just freeing of an
> > unused page and some kind of "sleep after first alloc fail, break after
> > second alloc fail" logic.
> > This could now be moved to khugepaged_do_scan() loop itself and maybe it
> > will be easier to follow.
> 
> Aha, I see. Misunderstood him. I'm supposed that you mean move into
> khugepaged_scan_mm_slot().

It may not be possible, but I'd always imagined that a cleanup of this
kind would get rid of all those "struct page **hpage" artifacts.

Hugh