Re: [v2 PATCH] fs/proc: task_mmu.c: don't read mapcount for migration entry

[Yang Shi]

On Fri, Jan 28, 2022 at 12:02 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 27.01.22 22:16, Yang Shi wrote:
> > On Wed, Jan 26, 2022 at 10:54 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
> >>
> >>>>> Just page lock or elevated page refcount could serialize against THP
> >>>>> split AFAIK.
> >>>>>
> >>>>>>
> >>>>>> But yeah, using the mapcount of a page that is not even mapped
> >>>>>> (migration entry) is clearly wrong.
> >>>>>>
> >>>>>> To summarize: reading the mapcount on an unlocked page will easily
> >>>>>> return a wrong result and the result should not be relied upon. reading
> >>>>>> the mapcount of a migration entry is dangerous and certainly wrong.
> >>>>>
> >>>>> Depends on your usecase. Some just want to get a snapshot, just like
> >>>>> smaps, they don't care.
> >>>>
> >>>> Right, but as discussed, even the snapshot might be slightly wrong. That
> >>>> might be just fine for smaps (and I would have enjoyed a comment in the
> >>>> code stating that :) ).
> >>>
> >>> I think that is documented already, see Documentation/filesystems/proc.rst:
> >>>
> >>> Note: reading /proc/PID/maps or /proc/PID/smaps is inherently racy (consistent
> >>> output can be achieved only in the single read call).
> >>
> >> Right, but I think there is a difference between
> >>
> >> * Atomic values that change immediately afterwards ("this value used to
> >>   be true at one point in time")
> >> * Values that are unstable because we cannot read them atomically ("this
> >>   value never used to be true")
> >>
> >> I'd assume with the documented race we actually talk about the first
> >> point, but I might be just wrong.
> >>
> >>>
> >>> Of course, if the extra note is preferred in the code, I could try to
> >>> add some in a separate patch.
> >>
> >> When staring at the (original) code I would have hoped to find something
> >> like:
> >>
> >> /*
> >>  * We use page_mapcount() to get a snapshot of the mapcount. Without
> >>  * holding the page lock this snapshot can be slightly wrong as we
> >>  * cannot always read the mapcount atomically. As long we hold the PT
> >>  * lock, the page cannot get unmapped and it's at safe to call
> >>  * page_mapcount().
> >>  */
> >>
> >> With the addition of
> >>
> >> "... For unmapped pages (e.g., migration entries) we cannot guarantee
> >> that, so treat the mapcount as being 1."
> >
> > It seems a little bit confusing to me, it is not safe to call with PTL
> > held either, right? I'd like to rephrase the note to:
>
> The implication that could have been spelled out is that only a mapped
> page can get unmapped. (I know, there are some weird migration entries
> nowadays ...)

Yes, I see your point. Just felt "only a mapped page can get unmapped"
sounds not that straightforward (just my personal feeling). How's
about "It is not safe to call page_mapcount() even with PTL held if
the page is not mapped, especially for migration entries".

>
> /*
>  * We use page_mapcount() to get a snapshot of the mapcount. Without
>  * holding the page lock this snapshot can be slightly wrong as we
>  * cannot always read the mapcount atomically. As long we hold the PT
>  * lock, a mapped page cannot get unmapped and it's at safe to call
>  * page_mapcount(). Especially for migration entries, it's not safe to
>  * call page_mapcount(), so we treat the mapcount as being 1.
>  */
>
>
>
> --
> Thanks,
>
> David / dhildenb
>