Re: [PATCH v2] After swapout/swapin private dirty mappings are reported clean in smaps

Matt Mackall
2010-09-15T19:46:19Z

On Wed, 2010-09-15 at 12:18 -0700, Hugh Dickins wrote:
> On Wed, Sep 15, 2010 at 10:24 AM, Matt Mackall <mpm@selenic.com> wrote:
>
> > But that's my point: the consistency problem is NOT in smaps. The page
> > is NOT marked dirty, ergo smaps doesn't report it as dirty. Whether or
> > not there is MORE information smaps could be reporting is irrelevant,
> > the information it IS reporting is consistent with the underlying VM
> > data. If there's an inconsistency about what it means to be clean, it's
> > either in the VM or in your head.
> >
> > And I frankly think it's in the VM.
>
> I don't believe there's any problem in the VM here, we'd be having
> SIGSEGVs all over if there were.

Of course it works. It's just not as orthogonal (aka consistent) as it
could be in this case: it's not actually reflecting any of the usual
meanings of dirtiness here.

> The problem is that /proc/pid/smaps exports a simplified view of the
> VM, and Richard and Nikanth were hoping that it gave them some info
> which it has never pretended to give them,
>
> It happens to use a pte_dirty(ptent) test: you could argue that that
> should be pte_dirty(ptent) || PageDirty(page) (which would then "fix
> the issue" which Richard sees with swapoff/swapon),

That might be interesting. Are there any other notable cases where
pte_dirty() differs from PageDirty()?

> or you could argue
> that it should be pte_dirty(ptent) || PageDirty(page) ||
> PageSwapCache(page) (which would then note clean copies of swap cache
> as dirty in the sense which Richard and Nikanth are interested in).
>
> But after these years, we should probably assume that most users of
> /proc/pid/smaps are used to the existing pte_dirty(ptent) test, and
> would be troubled by a departure from it.

Dunno, my smem tool[1] is probably also expecting too much here and I
should know better!

> > In any case, I don't think Nikanth's fix is the right fix, as it
> > basically says "you can't trust any of this". Either swap should return
> > the pages to their pre-swap dirty state in the VM, or we should add
> > another field here:
> >
> > Weird_Anon_Page_You_Should_Pretend_Is_Private_Dirty: 8 kB
>
> I think that the most widely useful but simple extension of
> /proc/pid/smaps, that would give them the info they want, would indeed
> be to counts ptes pointing to PageAnon pages and report that total on
> an additional line (say, just before "Swap:"); but there's no need for
> the derogatory name you propose there, "Anon:" would suit fine!

Yes, that wasn't a serious suggestion.

[1] http://www.selenic.com/smem/
--
Mathematics is the supreme nostalgia of our time.