Re: [v2 PATCH 2/2] mm: mempolicy: handle vma with unmovable pages mapped correctly in mbind

[Yang Shi]

On 7/17/19 11:50 AM, Vlastimil Babka wrote:
> On 7/17/19 8:23 PM, Yang Shi wrote:
> > On 7/16/19 10:28 AM, Yang Shi wrote:
> > > On 7/16/19 5:07 AM, Vlastimil Babka wrote:
> > > > On 6/22/19 2:20 AM, Yang Shi wrote:
> > > > > @@ -969,10 +975,21 @@ static long do_get_mempolicy(int *policy,
> > > > > nodemask_t *nmask,
> > > > >  Â /*
> > > > >  ÂÂ * page migration, thp tail pages can be passed.
> > > > >  ÂÂ */
> > > > > -static void migrate_page_add(struct page *page, struct list_head
> > > > > *pagelist,
> > > > > +static int migrate_page_add(struct page *page, struct list_head
> > > > > *pagelist,
> > > > >  ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ unsigned long flags)
> > > > >  Â {
> > > > >  ÂÂÂÂÂ struct page *head = compound_head(page);
> > > > > +
> > > > > +ÂÂÂ /*
> > > > > + * Non-movable page may reach here. And, there may be
> > > > > +ÂÂÂÂ * temporaty off LRU pages or non-LRU movable pages.
> > > > > +ÂÂÂÂ * Treat them as unmovable pages since they can't be
> > > > > + * isolated, so they can't be moved at the moment. It
> > > > > +ÂÂÂÂ * should return -EIO for this case too.
> > > > > +ÂÂÂÂ */
> > > > > +ÂÂÂ if (!PageLRU(head) && (flags & MPOL_MF_STRICT))
> > > > > +ÂÂÂÂÂÂÂ return -EIO;
> > > > > +
> > > > Hm but !PageLRU() is not the only way why queueing for migration can
> > > > fail, as can be seen from the rest of the function. Shouldn't all cases
> > > > be reported?
> > > Do you mean the shared pages and isolation failed pages? I'm not sure
> > > whether we should consider these cases break the semantics or not, so
> > > I leave them as they are. But, strictly speaking they should be
> > > reported too, at least for the isolation failed page.
> CC'd linux-api, should be done on v3 posting also.
>
> > By reading mbind man page, it says:
> >
> > If MPOL_MF_MOVE is specified in flags, then the kernel will attempt to
> > move all the existing pages in the memory range so that they follow the
> > policy. Pages that are shared with other processes will not be moved.
> > If MPOL_MF_STRICT is also specified, then the call fails with the error
> > EIO if some pages could not be moved.
> I don't think this means that for shared pages, -EIO should not be
> reported. I can imagine both interpretations of the paragraph. I guess
> we can be conservative and keep not reporting them, if that was always
> the case - but then perhaps clarify the man page?

Yes, I agree the man page does looks ambiguous. Anyway, I think we 
could add a patch later to kernel or manpage for either interpretations 
once it gets clarified.

> > It looks the code already handles shared page correctly, we just need
> > return -EIO for isolation failed page if MPOL_MF_STRICT is specified.
> >
> > > Thanks,
> > > Yang
> > >
> > > > >  ÂÂÂÂÂ /*
> > > > >  ÂÂÂÂÂÂ * Avoid migrating a page that is shared with others.
> > > > >  ÂÂÂÂÂÂ */
> > > > > @@ -984,6 +1001,8 @@ static void migrate_page_add(struct page *page,
> > > > > struct list_head *pagelist,
> > > > >  ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ hpage_nr_pages(head));
> > > > >  ÂÂÂÂÂÂÂÂÂ }
> > > > >  ÂÂÂÂÂ }
> > > > > +
> > > > > +ÂÂÂ return 0;
> > > > >  Â }
> > > > >  Â Â /* page allocation callback for NUMA node migration */
> > > > > @@ -1186,9 +1205,10 @@ static struct page *new_page(struct page
> > > > > *page, unsigned long start)
> > > > >  Â }
> > > > >  Â #else
> > > > >  Â -static void migrate_page_add(struct page *page, struct list_head
> > > > > *pagelist,
> > > > > +static int migrate_page_add(struct page *page, struct list_head
> > > > > *pagelist,
> > > > >  ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ unsigned long flags)
> > > > >  Â {
> > > > > +ÂÂÂ return -EIO;
> > > > >  Â }
> > > > >  Â Â int do_migrate_pages(struct mm_struct *mm, const nodemask_t *from,