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SubjectRe: [RFC 2/2] cma: support MIGRATE_DISCARD
Hi Rik,

On Wed, Aug 15, 2012 at 02:58:01PM -0400, Rik van Riel wrote:
> On 08/14/2012 04:57 AM, Minchan Kim wrote:
> >This patch introudes MIGRATE_DISCARD mode in migration.
> >It drop clean cache pages instead of migration so that
> >migration latency could be reduced. Of course, it could
> >evict code pages but latency of big contiguous memory
> >is more important than some background application's slow down
> >in mobile embedded enviroment.
>
> Would it be an idea to only drop clean UNMAPPED
> page cache pages?

Firstly I thougt about that but I chose more agressive thing.
Namely, even drop mapped page cache.
Because it can reduce latency more(ex, memcpy + remapping cost
during migration) and it could not trivial if migration range is big.

>
> >Signed-off-by: Minchan Kim <minchan@kernel.org>
>
> >@@ -799,12 +802,39 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
> > goto skip_unmap;
> > }
> >
> >+ file = page_is_file_cache(page);
> >+ ttu_flags = TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS;
> >+
> >+ if (!(mode & MIGRATE_DISCARD) || !file || PageDirty(page))
> >+ ttu_flags |= TTU_MIGRATION;
> >+ else
> >+ discard_mode = true;
> >+
> > /* Establish migration ptes or remove ptes */
> >- try_to_unmap(page, TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS);
> >+ try_to_unmap(page, ttu_flags);
>
> This bit looks wrong, because you end up ignoring
> mlock and then discarding the page.

Argh, Thanks!
I will fix it in next spin.

>
> Only dropping clean page cache pages that are not
> mapped would avoid that problem, without introducing
> much complexity in the code.

Hmm, I don't think it makes code much complex.
How about this?

diff --git a/mm/rmap.c b/mm/rmap.c
index 0f3b7cd..0909d79 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1223,7 +1223,8 @@ out:
* repeatedly from try_to_unmap_ksm, try_to_unmap_anon or try_to_unmap_file.
*/
int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
- unsigned long address, enum ttu_flags flags)
+ unsigned long address, enum ttu_flags flags,
+ unsigned long *vm_flags)
{
struct mm_struct *mm = vma->vm_mm;
pte_t *pte;
@@ -1235,6 +1236,7 @@ int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
if (!pte)
goto out;

+ vm_flags |= vma->vm_flags;
/*
* If the page is mlock()d, we cannot swap it out.
* If it's recently referenced (perhaps page_referenced
@@ -1652,7 +1654,7 @@ out:
* SWAP_FAIL - the page is unswappable
* SWAP_MLOCK - page is mlocked.
*/
-int try_to_unmap(struct page *page, enum ttu_flags flags)
+int try_to_unmap(struct page *page, enum ttu_flags flags, unsigned long *vm_flags)
{
int ret;

<snip>
+ file = page_is_file_cache(page);
+ ttu_flags = TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS;
+
+ if (!(mode & MIGRATE_DISCARD) || !file || PageDirty(page) ||
+ vm_flags & VM_LOCKED)
+ ttu_flags |= TTU_MIGRATION;
+ else
+ discard_mode = true;
+

>
> That would turn the test above into:
>
> if (!page_mapped(page))
> discard_mode = true;
>
> > skip_unmap:
> >- if (!page_mapped(page))
> >- rc = move_to_new_page(newpage, page, remap_swapcache, mode);
> >+ if (!page_mapped(page)) {
> >+ if (!discard_mode)
> >+ rc = move_to_new_page(newpage, page, remap_swapcache, mode);
> >+ else {
> >+ struct address_space *mapping;
> >+ mapping = page_mapping(page);
> >+
> >+ if (page_has_private(page)) {
> >+ if (!try_to_release_page(page, GFP_KERNEL)) {
> >+ rc = -EAGAIN;
> >+ goto uncharge;
> >+ }
> >+ }
> >+
> >+ if (remove_mapping(mapping, page))
> >+ rc = 0;
> >+ else
> >+ rc = -EAGAIN;
> >+ goto uncharge;
> >+ }
> >+ }
>
> This big piece of code could probably be split out
> into its own function.
>
>
>
> --
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--
Kind regards,
Minchan Kim


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