Re: [PATCH v2] mm: compaction: skip memory hole rapidly when isolating migratable pages

[David Hildenbrand]

On 15.06.23 09:22, Huang, Ying wrote:
> Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> writes:
>
> > On 6/15/2023 11:22 AM, Huang, Ying wrote:
> > > Hi, Mel,
> > > Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> writes:
> > >
> > > > On Tue, Jun 13, 2023 at 04:55:04PM +0800, Baolin Wang wrote:
> > > > > On some machines, the normal zone can have a large memory hole like
> > > > > below memory layout, and we can see the range from 0x100000000 to
> > > > > 0x1800000000 is a hole. So when isolating some migratable pages, the
> > > > > scanner can meet the hole and it will take more time to skip the large
> > > > > hole. From my measurement, I can see the isolation scanner will take
> > > > > 80us ~ 100us to skip the large hole [0x100000000 - 0x1800000000].
> > > > >
> > > > > So adding a new helper to fast search next online memory section
> > > > > to skip the large hole can help to find next suitable pageblock
> > > > > efficiently. With this patch, I can see the large hole scanning only
> > > > > takes < 1us.
> > > > >
> > > > > [    0.000000] Zone ranges:
> > > > > [    0.000000]   DMA      [mem 0x0000000040000000-0x00000000ffffffff]
> > > > > [    0.000000]   DMA32    empty
> > > > > [    0.000000]   Normal   [mem 0x0000000100000000-0x0000001fa7ffffff]
> > > > > [    0.000000] Movable zone start for each node
> > > > > [    0.000000] Early memory node ranges
> > > > > [    0.000000]   node   0: [mem 0x0000000040000000-0x0000000fffffffff]
> > > > > [    0.000000]   node   0: [mem 0x0000001800000000-0x0000001fa3c7ffff]
> > > > > [    0.000000]   node   0: [mem 0x0000001fa3c80000-0x0000001fa3ffffff]
> > > > > [    0.000000]   node   0: [mem 0x0000001fa4000000-0x0000001fa402ffff]
> > > > > [    0.000000]   node   0: [mem 0x0000001fa4030000-0x0000001fa40effff]
> > > > > [    0.000000]   node   0: [mem 0x0000001fa40f0000-0x0000001fa73cffff]
> > > > > [    0.000000]   node   0: [mem 0x0000001fa73d0000-0x0000001fa745ffff]
> > > > > [    0.000000]   node   0: [mem 0x0000001fa7460000-0x0000001fa746ffff]
> > > > > [    0.000000]   node   0: [mem 0x0000001fa7470000-0x0000001fa758ffff]
> > > > > [    0.000000]   node   0: [mem 0x0000001fa7590000-0x0000001fa7ffffff]
> > > > >
> > > > > Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> > > >
> > > > This may only be necessary for non-contiguous zones so a check for
> > > > zone_contiguous could be made but I suspect the saving, if any, would be
> > > > marginal.
> > > >
> > > > However, it's subtle that block_end_pfn can end up in an arbirary location
> > > > past the end of the zone or past cc->free_pfn. As the "continue" will update
> > > > cc->migrate_pfn, that might lead to errors in the future. It would be a
> > > > lot safer to pass in cc->free_pfn and do two things with the value. First,
> > > > there is no point scanning for a valid online section past cc->free_pfn so
> > > > terminating after cc->free_pfn may save some cycles. Second, cc->migrate_pfn
> > > > does not end up with an arbitrary value which is a more defensive approach
> > > > to any future programming errors.
> > > I have thought about this before.  Originally, I had thought that we
> > > were safe because cc->free_pfn should be in a online section and
> > > block_end_pfn should reach cc->free_pfn before the end of zone.  But
> > > after checking more code and thinking about it again, I found that the
> > > underlying sections may go offline under us during compaction.  So that,
> > > cc->free_pfn may be in a offline section or after the end of zone.  So,
> > > you are right, we need to consider the range of block_end_pfn.
> > > But, if we thought in this way (memory online/offline at any time),
> > > it
> > > appears that we need to check whether the underlying section was
> > > offlined.  For example, is it safe to use "pfn_to_page()" in
> > > "isolate_migratepages_block()"?  Is it possible for the underlying
> > > section to be offlined under us?
> >
> > It is possible. There is a previous discussion[1] about the race
> > between pfn_to_online_page() and memory offline.
> >
> > [1]
> > https://lore.kernel.org/lkml/87zgc6buoq.fsf@xxxxxxxxxx/T/#m642d91bcc726437e1848b295bc57ce249c7ca399
>
> Thank you very much for sharing!  That answers my questions directly!

I remember another discussion (but can't find it) regarding why memory 
compaction can get away without pfn_to_online_page() all over the place. 
The use is limited to __reset_isolation_pfn().

But yes, in theory pfn_to_online_page() can race with memory offlining.

--
Cheers,

David / dhildenb