Re: [PATCH 2/6] mm: mlocking in try_to_unmap_one

KOSAKI Motohiro
2009-11-13T08:26:17Z

> > On Wed, 11 Nov 2009, KOSAKI Motohiro wrote:
> >
> > Though it doesn't quite answer your question,
> > I'll just reinsert the last paragraph of my description here...
> >
> > > > try_to_unmap_file()'s TTU_MUNLOCK nonlinear handling was particularly
> > > > amusing: once unravelled, it turns out to have been choosing between
> > > > two different ways of doing the same nothing. Ah, no, one way was
> > > > actually returning SWAP_FAIL when it meant to return SWAP_SUCCESS.
> >
> > ...
> > > > @@ -1081,45 +1053,23 @@ static int try_to_unmap_file(struct page
> > ...
> > > >
> > > > - if (list_empty(&mapping->i_mmap_nonlinear))
> > > > + /* We don't bother to try to find the munlocked page in nonlinears */
> > > > + if (MLOCK_PAGES && TTU_ACTION(flags) == TTU_MUNLOCK)
> > > > goto out;
> > >
> > > I have dumb question.
> > > Does this shortcut exiting code makes any behavior change?
> >
> > Not dumb. My intention was to make no behaviour change with any of
> > this patch; but in checking back before completing the description,
> > I suddenly realized that that shortcut intentionally avoids the
> >
> > if (max_nl_size == 0) { /* all nonlinears locked or reserved ? */
> > ret = SWAP_FAIL;
> > goto out;
> > }
> >
> > (which doesn't show up in the patch: you'll have to look at rmap.c),
> > which used to have the effect of try_to_munlock() returning SWAP_FAIL
> > in the case when there were one or more VM_NONLINEAR vmas of the file,
> > but none of them (and none of the covering linear vmas) VM_LOCKED.
> >
> > That should have been a SWAP_SUCCESS case, or with my changes
> > another SWAP_AGAIN, either of which would make munlock_vma_page()
> > count_vm_event(UNEVICTABLE_PGMUNLOCKED);
> > which would be correct; but the SWAP_FAIL meant that count was not
> > incremented in this case.
>
> Ah, correct.
> Then, we lost the capability unevictability of non linear mapping pages, right.
> if so, following additional patch makes more consistent?

[indistinct muttering]

Probably we can remove VM_NONLINEAR perfectly. I've never seen real user of it.