Re: [PATCH v3] mm, page_alloc: disallow __GFP_COMP in alloc_pages_exact()

[Mel Gorman <mgorman@techsingularity.net>]

On Mon, Mar 18, 2019 at 01:21:59PM +0100, Vlastimil Babka wrote:
> OK here's a new version that changes the patch to remove __GFP_COMP per
> the v2 discussion, and also fixes the bug Kirill spotted (thanks!).
> 
> ----8<----
> From 1fbc84c208573b885f51818ed823f89b3aa1e0ae Mon Sep 17 00:00:00 2001
> From: Vlastimil Babka <vbabka@suse.cz>
> Date: Thu, 14 Mar 2019 10:19:30 +0100
> Subject: [PATCH v3] mm, page_alloc: disallow __GFP_COMP in alloc_pages_exact()
> 
> 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 may have really
> 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 2, because as Michal Hocko put it: "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@suse.cz>

Acked-by: Mel Gorman <mgorman@techsingularity.net>