Re: [RFC PATCH] mm: don't promote exclusive file folios of dying processes

[Johannes Weiner]

On Wed, Apr 16, 2025 at 11:40:31AM +0200, David Hildenbrand wrote:
> On 16.04.25 11:38, Barry Song wrote:
> > On Wed, Apr 16, 2025 at 5:32 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
> >>
> >> On 16.04.25 11:24, Barry Song wrote:
> >>> On Wed, Apr 16, 2025 at 4:32 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
> >>>>
> >>>> On 12.04.25 10:58, Barry Song wrote:
> >>>>> From: Barry Song <v-songbaohua@xxxxxxxx>
> >>>>>
> >>>>> Promoting exclusive file folios of a dying process is unnecessary and
> >>>>> harmful. For example, while Firefox is killed and LibreOffice is
> >>>>> launched, activating Firefox's young file-backed folios makes it
> >>>>> harder to reclaim memory that LibreOffice doesn't use at all.
> >>>>
> >>>> Do we know when it is reasonable to promote any folios of a dying process?
> >>>>
> >>>
> >>> I don't know. It seems not reasonable at all. if one service crashes due to
> >>> SW bug, systemd will restart it immediately. this might be the case promoting
> >>> folios might be good. but it is really a bug of the service, not a normal case.
> >>>
> >>>> Assume you restart Firefox, would it really matter to promote them when
> >>>> unmapping? New Firefox would fault-in / touch the ones it really needs
> >>>> immediately afterwards?
> >>>
> >>> Usually users kill firefox to start other applications (users intend
> >>> to free memory
> >>> for new applications). For Android, an app might be killed because it has been
> >>> staying in the background inactively for a while.
> >>
> >>> On the other hand, even if users restart firefox immediately, their folios are
> >>> probably still in LRU to hit.
> >>
> >> Right, that's what I'm thinking.
> >>
> >> So I wonder if we could just say "the whole process is going down; even
> >> if we had some recency information, that could only affect some other
> >> process, where we would have to guess if it really matters".
> >>
> >> If the data is important, one would assume that another process would
> >> soon access it either way, and as you say, likely it will still be on
> >> the LRU to hit.
> > 
> > I'll include this additional information in the v2 version of the patch since
> > you think it would be helpful.
> > 
> > Regarding the exclusive flag - I'm wondering whether we actually need to
> > distinguish between exclusive and shared folios in this case. The current
> > patch uses the exclusive flag mainly to reduce controversy, but even for
> > shared folios: does the recency from a dying process matter? The
> > recency information only reflects the dying process's usage pattern, which
> > will soon be irrelevant.
> 
> Exactly my thoughts. So if we can simplify -- ignore it completely -- 
> that would certainly be nice.

This doesn't sound right to me.

Remembering the accesses of an exiting task is very much the point of
this. Consider executables and shared libraries repeatedly referenced
by short-lived jobs, like shell scripts, compiles etc.

MADV_COLD and MADV_PAGEOUT where specifically added for this Android
usecase - the rare situation where you *know* those pages are done.