Re: [PATCH 4/9] mm/compaction: simplify pfn iteration in isolate_freepages_range

[Baolin Wang]

On 8/15/2023 5:32 PM, Kemeng Shi wrote:
> on 8/15/2023 4:38 PM, Baolin Wang wrote:
> > On 8/5/2023 7:07 PM, Kemeng Shi wrote:
> > > We call isolate_freepages_block in strict mode, continuous pages in
> > > pageblock will be isolated if isolate_freepages_block successed.
> > > Then pfn + isolated will point to start of next pageblock to scan
> > > no matter how many pageblocks are isolated in isolate_freepages_block.
> > > Use pfn + isolated as start of next pageblock to scan to simplify the
> > > iteration.
> >
> > IIUC, the isolate_freepages_block() can isolate high-order free pages, which means the pfn + isolated can be larger than the block_end_pfn. So in your patch, the 'block_start_pfn' and 'block_end_pfn' can be in different pageblocks, that will break pageblock_pfn_to_page().
> In for update statement, we always update block_start_pfn to pfn and

I mean, you changed to:
1) pfn += isolated;
2) block_start_pfn = pfn;
3) block_end_pfn = pfn + pageblock_nr_pages;

But in 1) pfn + isolated can go outside of the currnet pageblock if 
isolating a high-order page, for example, located in the middle of the 
next pageblock. So that the block_start_pfn can point to the middle of 
the next pageblock, not the start position. Meanwhile after 3), the 
block_end_pfn can point another pageblock. Or I missed something else?

> update block_end_pfn to pfn + pageblock_nr_pages. So they should point
> to the same pageblock. I guess you missed the change to update of
> block_end_pfn. :)
> > > Signed-off-by: Kemeng Shi <shikemeng@xxxxxxxxxxxxxxx>
> > > ---
> > >    mm/compaction.c | 14 ++------------
> > >    1 file changed, 2 insertions(+), 12 deletions(-)
> > >
> > > diff --git a/mm/compaction.c b/mm/compaction.c
> > > index 684f6e6cd8bc..8d7d38073d30 100644
> > > --- a/mm/compaction.c
> > > +++ b/mm/compaction.c
> > > @@ -733,21 +733,11 @@ isolate_freepages_range(struct compact_control *cc,
> > >        block_end_pfn = pageblock_end_pfn(pfn);
> > >          for (; pfn < end_pfn; pfn += isolated,
> > > -                block_start_pfn = block_end_pfn,
> > > -                block_end_pfn += pageblock_nr_pages) {
> > > +                block_start_pfn = pfn,
> > > +                block_end_pfn = pfn + pageblock_nr_pages) {
> > >            /* Protect pfn from changing by isolate_freepages_block */
> > >            unsigned long isolate_start_pfn = pfn;
> > >    -        /*
> > > -         * pfn could pass the block_end_pfn if isolated freepage
> > > -         * is more than pageblock order. In this case, we adjust
> > > -         * scanning range to right one.
> > > -         */
> > > -        if (pfn >= block_end_pfn) {
> > > -            block_start_pfn = pageblock_start_pfn(pfn);
> > > -            block_end_pfn = pageblock_end_pfn(pfn);
> > > -        }
> > > -
> > >            block_end_pfn = min(block_end_pfn, end_pfn);
> > >              if (!pageblock_pfn_to_page(block_start_pfn,