linux-kernel - Re: [PATCH v2 0/2] fix MADV_COLLAPSE issue if THP settings are disabled

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Message-ID: <bb496158-5915-40a2-ab7c-0039a28600af@lucifer.local>
Date: Fri, 13 Jun 2025 15:42:06 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Usama Arif <usamaarif642@...il.com>
Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>, akpm@...ux-foundation.org,
        hughd@...gle.com, david@...hat.com, Liam.Howlett@...cle.com,
        npache@...hat.com, ryan.roberts@....com, dev.jain@....com,
        ziy@...dia.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/2] fix MADV_COLLAPSE issue if THP settings are
 disabled

On Fri, Jun 13, 2025 at 03:39:33PM +0100, Usama Arif wrote:
>
>
> On 13/06/2025 15:29, Lorenzo Stoakes wrote:
> > On Fri, Jun 13, 2025 at 03:23:19PM +0100, Usama Arif wrote:
> >>
> >>
> >> On 05/06/2025 09:00, Baolin Wang wrote:
> >>> As we discussed in the previous thread [1], the MADV_COLLAPSE will ignore
> >>> the system-wide anon/shmem THP sysfs settings, which means that even though
> >>> we have disabled the anon/shmem THP configuration, MADV_COLLAPSE will still
> >>> attempt to collapse into a anon/shmem THP. This violates the rule we have
> >>> agreed upon: never means never. This patch set will address this issue.
> >>
> >> Hi Baolin,
> >>
> >> I know never means never, but I also thought that the per-size toggles had
> >> priority over the system ones. This was discussed in [1] as well.
> >>
> >> My understanding with these patches is that if we have:
> >>
> >> [root@vm4 vmuser]# cat /sys/kernel/mm/transparent_hugepage/enabled
> >> always madvise [never]
> >> [root@vm4 vmuser]# cat /sys/kernel/mm/transparent_hugepage/hugepages-2048kB/enabled
> >> always inherit [madvise] never
> >>
> >> Than without these patches we get a hugepage when we do MADV_HUGEPAGE, but with
> >> these we won't get a hugepage anymore eventhough hugepages-2048kB/enabled is set
> >> to madvise?
> >
> > This isn't correct, madvise at a specific pagesize will still be permitted for
> > MADV_COLLAPSE.
> >
> > In current contender for this patch:
> >
> > /* Strictly mask requested anonymous orders according to sysfs settings. */
> > static inline unsigned long __thp_mask_anon_orders(unsigned long vm_flags,
> >                 unsigned long tva_flags, unsigned long orders)
> > {
> >         const unsigned long always = READ_ONCE(huge_anon_orders_always);
> >         const unsigned long madvise = READ_ONCE(huge_anon_orders_madvise);
> >         const unsigned long inherit = READ_ONCE(huge_anon_orders_inherit);;
> >         const unsigned long never = ~(always | madvise | inherit);
> >
> > Note that madvise is considered here.
> >
>
> Ah ok, Thanks for clearing that! I was reviewing the original patch in [1] but I
> see this version in the replies.
>
> I wish this function was simpler :) or maybe its me that takes so much time
> to figure out if the order will be set or not by the end of the function.

Couldn't agree more... this whole thing needs major work, it's massively confusing

>
> [1] https://lore.kernel.org/all/8eefb0809c598fadaa4a022634fba5689a4f3257.1749109709.git.baolin.wang@linux.alibaba.com/
>
> Thanks!
> Usama
>