On 3/14/19 11:15 AM, Michal Hocko wrote: > 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". I initially went with 2 as well, as you can see from v1 :) but then I looked at the commit [2] mentioned in [1] and I think ALSA legitimaly uses __GFP_COMP so that the pages are then mapped to userspace. Breaking that didn't seem good. The point is that with the warning in place, A developer will immediately know that they did something wrong, regardless if the size is power-of-two or not. But yeah, if it's adding of __GFP_COMP that is not deterministic, a bug can still sit silently for a while. But maybe we could go with 1) if free_pages_exact() is also adjusted to check for CompoundPage and free it properly? >> [1] https://lore.kernel.org/lkml/20181126002805.GI18977@shao2-debian/T/#u [2] https://git.kernel.org/pub/scm/linux/kernel/git/tiwai/sound.git/commit/?id=3a6d1980fe96dbbfe3ae58db0048867f5319cdbf >> >> 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 >
