hugetlb: do not clear hugetlb dtor until allocating vmemmap
Patch series "Fix hugetlb free path race with memory errors". In the discussion of Jiaqi Yan's series "Improve hugetlbfs read on HWPOISON hugepages" the race window was discovered. https://lore.kernel.org/linux-mm/20230616233447.GB7371@monkey/ Freeing a hugetlb page back to low level memory allocators is performed in two steps. 1) Under hugetlb lock, remove page from hugetlb lists and clear destructor 2) Outside lock, allocate vmemmap if necessary and call low level free Between these two steps, the hugetlb page will appear as a normal compound page. However, vmemmap for tail pages could be missing. If a memory error occurs at this time, we could try to update page flags non-existant page structs. A much more detailed description is in the first patch. The first patch addresses the race window. However, it adds a hugetlb_lock lock/unlock cycle to every vmemmap optimized hugetlb page free operation. This could lead to slowdowns if one is freeing a large number of hugetlb pages. The second path optimizes the update_and_free_pages_bulk routine to only take the lock once in bulk operations. The second patch is technically not a bug fix, but includes a Fixes tag and Cc stable to avoid a performance regression. It can be combined with the first, but was done separately make reviewing easier. This patch (of 2): 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. Link: https://lkml.kernel.org/r/20230711220942.43706-1-mike.kravetz@oracle.com Link: https://lkml.kernel.org/r/20230711220942.43706-2-mike.kravetz@oracle.com Fixes: ad2fa371 ("mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page") Signed-off-by:Mike Kravetz <mike.kravetz@oracle.com> Reviewed-by: