Re: [PATCH] mm/compaction: add check mechanism to avoid cma alloc fail

[Haiqiang Gong (龚海强)]

> Why the CMA memory can not be migrated? Could you describe in more 
> detail the reasons for the CMA memory migration failure?
When allocate for cma memory by module, the memory used by system in
the cma buffer will be migrate outside the cma buffer. If the refcount
of the current page in cma buffer is greater than the mapcount, the
migrate will fail.
The page that failed to be migrate was not originally in the cma
buffer, but was migrate in cma buffer after the kernel compaction.
After this memory is kcompacted and placed in the cma buffer, under 
certain special conditions, the refcount may be greater than the
mapcount(ex:the current page is being used by fs), and then migrate
will fail.


On Mon, 2024-01-22 at 14:59 +0800, Baolin Wang wrote:
>  	 
> External email : Please do not click links or open attachments until
> you have verified the sender or the content.
>  
> 
> On 1/22/2024 10:23 AM, Haiqiang Gong wrote:
> > cma alloc may fail when we doing cma alloc/free test on kernel
> 5.10/5.15.
> 
> Do you have a real use case for the cma alloc issue? And have you
> tried 
> it on the new kernel?
> 
> > We found that the next memory cannot be migrated because of the
> alloc of
> > fs as next backtrace:
> > __alloc_pages_nodemask
> > pagecache_get_page
> > grow_dev_page
> > __getblk_gfp
> > ext4_sb_breadahead_unmovable
> > __ext4_get_inode_loc
> > __ext4_iget
> > ext4_lookup
> > __lookup_slow
> > walk_component
> > path_lookupat
> > filename_lookup
> > vfs_statx
> > This kind of unmovable memory is not placed in the cma buffer when
> kernel
> > memory alloc but is migrated in by kcompactd when the kernel
> migration.
> > It will cause memory can't be migrate when cma alloc.
> 
> Why the CMA memory can not be migrated? Could you describe in more 
> detail the reasons for the CMA memory migration failure?
> 
> > Add check mechanism in the compaction_alloc() where kcompaced alloc
> for
> > memory. Will return NULL and give up this memory migration if the
> > allocated memory is in the cma buffer and the memory is unmovable.
> > 
> > Signed-off-by: Haiqiang Gong <Haiqiang.Gong@xxxxxxxxxxxx>
> > ---
> >   mm/compaction.c | 38 ++++++++++++++++++++++++++++++++++++++
> >   1 file changed, 38 insertions(+)
> > 
> > diff --git a/mm/compaction.c b/mm/compaction.c
> > index 27ada42924d5..29c0661adc22 100644
> > --- a/mm/compaction.c
> > +++ b/mm/compaction.c
> > @@ -25,6 +25,11 @@
> >   #include <linux/psi.h>
> >   #include "internal.h"
> >   
> > +#ifdef CONFIG_CMA
> > +#include <linux/cma.h>
> > +#include "cma.h"
> > +#endif
> > +
> >   #ifdef CONFIG_COMPACTION
> >   /*
> >    * Fragmentation score check interval for proactive compaction
> purposes.
> > @@ -1758,6 +1763,33 @@ static void isolate_freepages(struct
> compact_control *cc)
> >   split_map_pages(freelist);
> >   }
> >   
> > +#ifdef CONFIG_CMA
> > +static bool is_in_cma_range(struct folio *folio)
> > +{
> > +int i;
> > +unsigned long pfn = 0;
> > +struct page *page = folio_page(folio, 0);
> > +
> > +pfn = page_to_pfn(page);
> > +for (i = 0; i < cma_area_count; i++) {
> > +struct cma *cma = &cma_areas[i];
> > +
> > +if (cma->base_pfn <= pfn && (cma->base_pfn + cma->count) > pfn)
> > +return true;
> > +}
> > +
> > +return false;
> > +}
> > +
> > +static bool forbid_move_to_cma_range(struct folio *src, struct
> folio *dst)
> > +{
> > +if (folio_mapping(src) && is_in_cma_range(dst))
> > +return true;
> > +
> > +return false;
> > +}
> > +#endif
> > +
> >   /*
> >    * This is a migrate-callback that "allocates" freepages by
> taking pages
> >    * from the isolated freelists in the block we are migrating to.
> > @@ -1775,6 +1807,12 @@ static struct folio *compaction_alloc(struct
> folio *src, unsigned long data)
> >   }
> >   
> >   dst = list_entry(cc->freepages.next, struct folio, lru);
> > +#ifdef CONFIG_CMA
> > +if (forbid_move_to_cma_range(src, dst)) {
> > +pr_notice("kcompactd: could not move non-cma memory to cma
> buffer\n");
> > +return NULL;
> > +}
> > +#endif
> >   list_del(&dst->lru);
> >   cc->nr_freepages--;
> >