Re: [PATCH 1/2] hugetlb: Do not clear hugetlb dtor until allocating vmemmap

[Mike Kravetz]

On 07/12/23 16:03, Muchun Song wrote:
> 
> 
> > On Jul 12, 2023, at 06:09, Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
> > 
> > Freeing a hugetlb page and releasing base pages back to the underlying
> > allocator such as buddy or cma is performed in two steps:
> > - remove_hugetlb_folio() is called to remove the folio from hugetlb
> >  lists, get a ref on the page and remove hugetlb destructor.  This
> >  all must be done under the hugetlb lock.  After this call, the page
> >  can be treated as a normal compound page or a collection of base
> >  size pages.
> > - update_and_free_hugetlb_folio() is called to allocate vmemmap if
> >  needed and the free routine of the underlying allocator is called
> >  on the resulting page.  We can not hold the hugetlb lock here.
> > 
> > One issue with this scheme is that a memory error could occur between
> > these two steps.  In this case, the memory error handling code treats
> > the old hugetlb page as a normal compound page or collection of base
> > pages.  It will then try to SetPageHWPoison(page) on the page with an
> > error.  If the page with error is a tail page without vmemmap, a write
> > error will occur when trying to set the flag.
> > 
> > Address this issue by modifying remove_hugetlb_folio() and
> > update_and_free_hugetlb_folio() such that the hugetlb destructor is not
> > cleared until after allocating vmemmap.  Since clearing the destructor
> > requires holding the hugetlb lock, the clearing is done in
> > remove_hugetlb_folio() if the vmemmap is present.  This saves a
> > lock/unlock cycle.  Otherwise, destructor is cleared in
> > update_and_free_hugetlb_folio() after allocating vmemmap.
> > 
> > Note that this will leave hugetlb pages in a state where they are marked
> > free (by hugetlb specific page flag) and have a ref count.  This is not
> > a normal state.  The only code that would notice is the memory error
> > code, and it is set up to retry in such a case.
> > 
> > A subsequent patch will create a routine to do bulk processing of
> > vmemmap allocation.  This will eliminate a lock/unlock cycle for each
> > hugetlb page in the case where we are freeing a large number of pages.
> > 
> > Fixes: ad2fa3717b74 ("mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page")
> > Cc: <stable@xxxxxxxxxxxxxxx>
> > Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> 
> Hi Mike,
> 
> I have seen an issue proposed by Jiaqi Yan in [1]. I didn't see any
> resolution for it. Am I missing something with this fix?
> 
> [1] https://lore.kernel.org/linux-mm/CACw3F53iPiLrJt4pyaX2aaZ5BVg9tj8x_k6-v7=9Xn1nrh=UCw@xxxxxxxxxxxxxx/
> 

My mistake!  I sent the old version of the patch.

The new version was modified to simply check the destructor via
folio_test_hugetlb() in order to decide if it should be cleared.

I will send V2.  Sorry!
-- 
Mike Kravetz