Re: [PATCH] mm/hugetlb: Move vmf_anon_prepare upfront in hugetlb_wp

[Oscar Salvador <osalvador@suse.de>]

On Tue, May 21, 2024 at 11:56:54AM +0200, David Hildenbrand wrote:
> On 21.05.24 09:34, Oscar Salvador wrote:
> > hugetlb_wp calls vmf_anon_prepare() after having allocated a page, which
> > means that we might need to call restore_reserve_on_error() upon error.
> > vmf_anon_prepare() releases the vma lock before returning, but
> > restore_reserve_on_error() expects the vma lock to be held by the caller.
> > 
> > Fix it by calling vmf_anon_prepare() before allocating the page.
> > 
> > Signed-off-by: Oscar Salvador <osalvador@suse.de>
> > Fixes: 9acad7ba3e25 ("hugetlb: use vmf_anon_prepare() instead of anon_vma_prepare()")
> > ---
> > I did not hit this bug, I just spotted this because I was looking at hugetlb_wp
> > for some other reason. And I did not want to get creative to see if I could
> > trigger this so I could get a backtrace.
> > My assumption is that we could trigger this if 1) this was a shared mapping,
> > so no anon_vma and 2) we call in GUP code with FOLL_WRITE, which would cause
> > the FLAG_UNSHARE to be passed, so we will end up in hugetlb_wp().
> 
> FOLL_WRITE should never result in FLAG_UNSHARE.

You are right.
It was quite early when I looked at this and I managed to confuse myself when
reading hugetlb_follow_page_mask().
 
> The joy of hugetlb reservation code.
> 
> LGTM

thanks David!
 

-- 
Oscar Salvador
SUSE Labs