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

[David Hildenbrand]

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.

--
Cheers,

David / dhildenb