On 02/12/2018 02:20 PM, Mike Kravetz wrote: > start_isolate_page_range() is used to set the migrate type of a > page block to MIGRATE_ISOLATE while attempting to start a > migration operation. It is assumed that only one thread is > attempting such an operation, and due to the limited number of > callers this is generally the case. However, there are no > guarantees and it is 'possible' for two threads to operate on > the same range. I confirmed my suspicions that this is possible today. As a test, I created a large CMA area at boot time. I wrote some code to exercise large allocations and frees via cma_alloc()/cma_release(). At the same time, I just allocated and freed'ed gigantic pages via the sysfs interface. After a little bit of running, 'free memory' on the system went to zero. After 'stopping' the tests, I observed that most zone normal page blocks were marked as MIGRATE_ISOLATE. Hence 'not available'. As mentioned in the commit message, I doubt we will see this is normal operations. But, my testing confirms that it is possible. Therefore, we should consider a patch like this or some other form of mitigation even of we don't move forward with adding the new interface. -- Mike Kravetz > > Since start_isolate_page_range() is called at the beginning of > such operations, have it return -EBUSY if MIGRATE_ISOLATE is > already set. > > This will allow start_isolate_page_range to serve as a > synchronization mechanism and will allow for more general use > of callers making use of these interfaces. > > Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx> > --- > mm/page_alloc.c | 8 ++++---- > mm/page_isolation.c | 10 +++++++++- > 2 files changed, 13 insertions(+), 5 deletions(-) > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index 76c9688b6a0a..064458f317bf 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -7605,11 +7605,11 @@ static int __alloc_contig_migrate_range(struct compact_control *cc, > * @gfp_mask: GFP mask to use during compaction > * > * The PFN range does not have to be pageblock or MAX_ORDER_NR_PAGES > - * aligned, however it's the caller's responsibility to guarantee that > - * we are the only thread that changes migrate type of pageblocks the > - * pages fall in. > + * aligned. The PFN range must belong to a single zone. > * > - * The PFN range must belong to a single zone. > + * The first thing this routine does is attempt to MIGRATE_ISOLATE all > + * pageblocks in the range. Once isolated, the pageblocks should not > + * be modified by others. > * > * Returns zero on success or negative error code. On success all > * pages which PFN is in [start, end) are allocated for the caller and > diff --git a/mm/page_isolation.c b/mm/page_isolation.c > index 165ed8117bd1..e815879d525f 100644 > --- a/mm/page_isolation.c > +++ b/mm/page_isolation.c > @@ -28,6 +28,13 @@ static int set_migratetype_isolate(struct page *page, int migratetype, > > spin_lock_irqsave(&zone->lock, flags); > > + /* > + * We assume we are the only ones trying to isolate this block. > + * If MIGRATE_ISOLATE already set, return -EBUSY > + */ > + if (is_migrate_isolate_page(page)) > + goto out; > + > pfn = page_to_pfn(page); > arg.start_pfn = pfn; > arg.nr_pages = pageblock_nr_pages; > @@ -166,7 +173,8 @@ __first_valid_page(unsigned long pfn, unsigned long nr_pages) > * future will not be allocated again. > * > * start_pfn/end_pfn must be aligned to pageblock_order. > - * Returns 0 on success and -EBUSY if any part of range cannot be isolated. > + * Returns 0 on success and -EBUSY if any part of range cannot be isolated > + * or any part of the range is already set to MIGRATE_ISOLATE. > */ > int start_isolate_page_range(unsigned long start_pfn, unsigned long end_pfn, > unsigned migratetype, bool skip_hwpoisoned_pages) > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
