On Wednesday, 3 March 2021 9:08:15 AM AEDT Zi Yan wrote:
> On 26 Feb 2021, at 2:18, Alistair Popple wrote:
> > diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> > index 7f1ee411bd7b..77fa17de51d7 100644
> > --- a/include/linux/rmap.h
> > +++ b/include/linux/rmap.h
> > @@ -86,8 +86,6 @@ struct anon_vma_chain {
> > };
> >
> > enum ttu_flags {
> > - TTU_MIGRATION = 0x1, /* migration mode */
> > -
> > TTU_SPLIT_HUGE_PMD = 0x4, /* split huge PMD if any */
>
> It implies freeze in try_to_migrate() and no freeze in try_to_unmap(). I
think
> we need some comments here, above try_to_migrate(), and above try_to_unmap()
> to clarify the implication.
Sure. This confused me for a bit and I was initially tempted to leave
TTU_SPLIT_FREEZE as a separate mode flag but looking at what freeze actually
does it made sense to remove it because try_to_migrate() is for installing
migration entries (which is what freeze does) and try_to_unmap() just unmaps.
So I'll add some comments to that effect.
> > TTU_IGNORE_MLOCK = 0x8, /* ignore mlock */
> > TTU_IGNORE_HWPOISON = 0x20, /* corrupted page is recoverable */
> > @@ -96,7 +94,6 @@ enum ttu_flags {
> > * do a final flush if necessary */
> > TTU_RMAP_LOCKED = 0x80, /* do not grab rmap lock:
> > * caller holds it */
> > - TTU_SPLIT_FREEZE = 0x100, /* freeze pte under splitting thp */
> > };
> >
> > #ifdef CONFIG_MMU
> > @@ -193,6 +190,7 @@ static inline void page_dup_rmap(struct page *page,
bool compound)
> > int page_referenced(struct page *, int is_locked,
> > struct mem_cgroup *memcg, unsigned long *vm_flags);
> >
> > +bool try_to_migrate(struct page *page, enum ttu_flags flags);
> > bool try_to_unmap(struct page *, enum ttu_flags flags);
> >
> > /* Avoid racy checks */
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index d00b93dc2d9e..357052a4567b 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -2351,16 +2351,16 @@ void vma_adjust_trans_huge(struct vm_area_struct
*vma,
> >
> > static void unmap_page(struct page *page)
> > {
> > - enum ttu_flags ttu_flags = TTU_IGNORE_MLOCK |
> > - TTU_RMAP_LOCKED | TTU_SPLIT_HUGE_PMD;
> > + enum ttu_flags ttu_flags = TTU_RMAP_LOCKED | TTU_SPLIT_HUGE_PMD;
> > bool unmap_success;
> >
> > VM_BUG_ON_PAGE(!PageHead(page), page);
> >
> > if (PageAnon(page))
> > - ttu_flags |= TTU_SPLIT_FREEZE;
> > -
> > - unmap_success = try_to_unmap(page, ttu_flags);
> > + unmap_success = try_to_migrate(page, ttu_flags);
> > + else
> > + unmap_success = try_to_unmap(page, ttu_flags |
> > + TTU_IGNORE_MLOCK);
>
> I think we need a comment here about why anonymous pages need
try_to_migrate()
> and others need try_to_unmap().
Historically this comes from baa355fd3314 ("thp: file pages support for
split_huge_page()") which says:
"We don't setup migration entries. Just unmap pages. It helps handling cases
when i_size is in the middle of the page: no need handle unmap pages beyond
i_size manually."
But I'll add a comment here, thanks.
- Alistair
> Thanks.
>
> —
> Best Regards,
> Yan Zi
