Re: [PATCH v4 1/7] mm: page_alloc: avoid merging non-fallbackable pageblocks with others.

Re: [PATCH v4 1/7] mm: page_alloc: avoid merging non-fallbackable pageblocks with others.

[Mel Gorman]

On Wed, Jan 19, 2022 at 02:06:17PM -0500, Zi Yan wrote:
> From: Zi Yan <ziy@nvidia.com>
> 
> This is done in addition to MIGRATE_ISOLATE pageblock merge avoidance.
> It prepares for the upcoming removal of the MAX_ORDER-1 alignment
> requirement for CMA and alloc_contig_range().
> 
> MIGRARTE_HIGHATOMIC should not merge with other migratetypes like
> MIGRATE_ISOLATE and MIGRARTE_CMA[1], so this commit prevents that too.
> Also add MIGRARTE_HIGHATOMIC to fallbacks array for completeness.
> 
> [1] https://lore.kernel.org/linux-mm/20211130100853.GP3366@techsingularity.net/
> 
> Signed-off-by: Zi Yan <ziy@nvidia.com>
>
> <SNIP>
>
> @@ -2484,6 +2483,7 @@ static int fallbacks[MIGRATE_TYPES][3] = {
>  	[MIGRATE_UNMOVABLE]   = { MIGRATE_RECLAIMABLE, MIGRATE_MOVABLE,   MIGRATE_TYPES },
>  	[MIGRATE_MOVABLE]     = { MIGRATE_RECLAIMABLE, MIGRATE_UNMOVABLE, MIGRATE_TYPES },
>  	[MIGRATE_RECLAIMABLE] = { MIGRATE_UNMOVABLE,   MIGRATE_MOVABLE,   MIGRATE_TYPES },
> +	[MIGRATE_HIGHATOMIC] = { MIGRATE_TYPES }, /* Never used */
>  #ifdef CONFIG_CMA
>  	[MIGRATE_CMA]         = { MIGRATE_TYPES }, /* Never used */
>  #endif

If it's never used, why is it added?

Otherwise looks fine so

Acked-by: Mel Gorman <mgorman@techsingularity.net>

-- 
Mel Gorman
SUSE Labs
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Re: [PATCH v4 1/7] mm: page_alloc: avoid merging non-fallbackable pageblocks with others.

[Mel Gorman]

On Mon, Jan 24, 2022 at 11:12:07AM -0500, Zi Yan wrote:
> On 24 Jan 2022, at 9:02, Mel Gorman wrote:
> 
> > On Wed, Jan 19, 2022 at 02:06:17PM -0500, Zi Yan wrote:
> >> From: Zi Yan <ziy@nvidia.com>
> >>
> >> This is done in addition to MIGRATE_ISOLATE pageblock merge avoidance.
> >> It prepares for the upcoming removal of the MAX_ORDER-1 alignment
> >> requirement for CMA and alloc_contig_range().
> >>
> >> MIGRARTE_HIGHATOMIC should not merge with other migratetypes like
> >> MIGRATE_ISOLATE and MIGRARTE_CMA[1], so this commit prevents that too.
> >> Also add MIGRARTE_HIGHATOMIC to fallbacks array for completeness.
> >>
> >> [1] https://lore.kernel.org/linux-mm/20211130100853.GP3366@techsingularity.net/
> >>
> >> Signed-off-by: Zi Yan <ziy@nvidia.com>
> >>
> >> <SNIP>
> >>
> >> @@ -2484,6 +2483,7 @@ static int fallbacks[MIGRATE_TYPES][3] = {
> >>  	[MIGRATE_UNMOVABLE]   = { MIGRATE_RECLAIMABLE, MIGRATE_MOVABLE,   MIGRATE_TYPES },
> >>  	[MIGRATE_MOVABLE]     = { MIGRATE_RECLAIMABLE, MIGRATE_UNMOVABLE, MIGRATE_TYPES },
> >>  	[MIGRATE_RECLAIMABLE] = { MIGRATE_UNMOVABLE,   MIGRATE_MOVABLE,   MIGRATE_TYPES },
> >> +	[MIGRATE_HIGHATOMIC] = { MIGRATE_TYPES }, /* Never used */
> >>  #ifdef CONFIG_CMA
> >>  	[MIGRATE_CMA]         = { MIGRATE_TYPES }, /* Never used */
> >>  #endif
> >
> > If it's never used, why is it added?
> 
> Just to make the fallbacks list complete, since MIGRATE_CMA and
> MIGRATE_ISOLATE are in the list. Instead, I can remove MIGRATE_CMA and
> MIGRATE_ISOLATE. WDYT?
> 

It probably makes more sense to remove them or replace them with a comment
stating what migratetypes do not have a fallback list. Do it as a separate
patch that stands alone. It does not need to be part of this series.

-- 
Mel Gorman
SUSE Labs
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Re: [PATCH v4 3/7] mm: page_isolation: check specified range for unmovable pages

[David Hildenbrand]

On 02.02.22 13:18, Oscar Salvador wrote:
> On Wed, Jan 19, 2022 at 02:06:19PM -0500, Zi Yan wrote:
>> From: Zi Yan <ziy@nvidia.com>
>>
>> Enable set_migratetype_isolate() to check specified sub-range for
>> unmovable pages during isolation. Page isolation is done
>> at max(MAX_ORDER_NR_PAEGS, pageblock_nr_pages) granularity, but not all
>> pages within that granularity are intended to be isolated. For example,
>> alloc_contig_range(), which uses page isolation, allows ranges without
>> alignment. This commit makes unmovable page check only look for
>> interesting pages, so that page isolation can succeed for any
>> non-overlapping ranges.
> 
> Another thing that came to my mind.
> Prior to this patch, has_unmovable_pages() was checking on pageblock
> granularity, starting at pfn#0 of the pageblock.
> With this patch, you no longer check on pageblock granularity, which
> means you might isolate a pageblock, but some pages that sneaked in
> might actually be unmovable.
> 
> E.g:
> 
> Let's say you have a pageblock that spans (pfn#512,pfn#1024),
> and you pass alloc_contig_range() (pfn#514,pfn#1024).
> has_unmovable_pages() will start checking the pageblock at pfn#514,
> and so it will mis pfn#512 and pfn#513. Isn't that a problem, if those
> pfn turn out to be actually unmovable?

That's the whole idea for being able to allocate parts of an unmovable
pageblock that are movable.

If the first part is unmovable but the second part is movable, nothing
should stop us from trying to allocate the second part.

Of course, we want to remember the original migratetype of the
pageblock, to restore that after isolation -- otherwise we'll end up
converting all such pageblocks to MIGRATE_MOVABLE. The next patch does
that IIRC.

However, devil is in the detail, and I still have to review those parts
of this series.


Note that there are no current users of alloc_contig_range() that
allocate < MAX_ORDER - 1 -- except CMA, but for CMA we immediately exit
has_unmovable_pages() either way.

-- 
Thanks,

David / dhildenb

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