On Thu 14-03-19 10:42:49, Vlastimil Babka wrote: > alloc_pages_exact*() allocates a page of sufficient order and then splits it > to return only the number of pages requested. That makes it incompatible with > __GFP_COMP, because compound pages cannot be split. > > As shown by [1] things may silently work until the requested size (possibly > depending on user) stops being power of two. Then for CONFIG_DEBUG_VM, BUG_ON() > triggers in split_page(). Without CONFIG_DEBUG_VM, consequences are unclear. > > There are several options here, none of them great: > > 1) Don't do the spliting when __GFP_COMP is passed, and return the whole > compound page. However if caller then returns it via free_pages_exact(), > that will be unexpected and the freeing actions there will be wrong. > > 2) Warn and remove __GFP_COMP from the flags. But the caller wanted it, so > things may break later somewhere. > > 3) Warn and return NULL. However NULL may be unexpected, especially for > small sizes. > > This patch picks option 3, as it's best defined. The question is whether callers of alloc_pages_exact do have any fallback because if they don't then this is forcing an always fail path and I strongly suspect this is not really what users want. I would rather go with 2) because "callers wanted it" is much less probable than "caller is simply confused and more gfp flags is surely better than fewer". > [1] https://lore.kernel.org/lkml/20181126002805.GI18977@shao2-debian/T/#u > > Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx> > --- > Sent v1 before amending commit, sorry. > > mm/page_alloc.c | 15 ++++++++++++--- > 1 file changed, 12 insertions(+), 3 deletions(-) > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index 0b9f577b1a2a..dd3f89e8f88d 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -4752,7 +4752,7 @@ static void *make_alloc_exact(unsigned long addr, unsigned int order, > /** > * alloc_pages_exact - allocate an exact number physically-contiguous pages. > * @size: the number of bytes to allocate > - * @gfp_mask: GFP flags for the allocation > + * @gfp_mask: GFP flags for the allocation, must not contain __GFP_COMP > * > * This function is similar to alloc_pages(), except that it allocates the > * minimum number of pages to satisfy the request. alloc_pages() can only > @@ -4768,6 +4768,10 @@ void *alloc_pages_exact(size_t size, gfp_t gfp_mask) > unsigned long addr; > > addr = __get_free_pages(gfp_mask, order); > + > + if (WARN_ON_ONCE(gfp_mask & __GFP_COMP)) > + return NULL; > + > return make_alloc_exact(addr, order, size); > } > EXPORT_SYMBOL(alloc_pages_exact); > @@ -4777,7 +4781,7 @@ EXPORT_SYMBOL(alloc_pages_exact); > * pages on a node. > * @nid: the preferred node ID where memory should be allocated > * @size: the number of bytes to allocate > - * @gfp_mask: GFP flags for the allocation > + * @gfp_mask: GFP flags for the allocation, must not contain __GFP_COMP > * > * Like alloc_pages_exact(), but try to allocate on node nid first before falling > * back. > @@ -4785,7 +4789,12 @@ EXPORT_SYMBOL(alloc_pages_exact); > void * __meminit alloc_pages_exact_nid(int nid, size_t size, gfp_t gfp_mask) > { > unsigned int order = get_order(size); > - struct page *p = alloc_pages_node(nid, gfp_mask, order); > + struct page *p; > + > + if (WARN_ON_ONCE(gfp_mask & __GFP_COMP)) > + return NULL; > + > + p = alloc_pages_node(nid, gfp_mask, order); > if (!p) > return NULL; > return make_alloc_exact((unsigned long)page_address(p), order, size); > -- > 2.20.1 -- Michal Hocko SUSE Labs
