Re: [PATCH] hugetlb: Convert alloc_buddy_hugetlb_folio to use a folio

[Oscar Salvador <osalvador@suse.de>]

On Wed, Apr 03, 2024 at 10:17:45PM +0100, Matthew Wilcox wrote:
> I think hugetlb is being unnecessarily paranoid here, tbh.  Or maybe
> this part is just badly structured; if we're allocating a hugetlb folio,
> it should be fine for its refcount to be temporarily elevated by someone
> else.  Not sure I can figure out what's going on in
> alloc_and_dissolve_hugetlb_folio() though.
 
AFAICR, the problem comes when we need to remap the pages for vmemmap
optimization [1].
So, IIUC:

1) if someone comes around and grabs a refcount (say something doing
   speculative stuff)
2) we do the remapping
3) that someone who took the refcount, now does a put_page()
4) vmemmap no longer points to the old page but the new one, meaning
   that that 'put_page()' is done on the wrong page.

@Munchun: Did I get this right?

[1] https://lore.kernel.org/linux-mm/YupRjWRiz4lPo+y7@FVFYT0MHHV2J/


-- 
Oscar Salvador
SUSE Labs