Re: [PATCH v3] hugetlb: freeze allocated pages before creating hugetlb pages

[Muchun Song]

> On Sep 22, 2022, at 04:27, Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
> 
> When creating hugetlb pages, the hugetlb code must first allocate
> contiguous pages from a low level allocator such as buddy, cma or
> memblock.  The pages returned from these low level allocators are
> ref counted.  This creates potential issues with other code taking
> speculative references on these pages before they can be transformed to
> a hugetlb page.  This issue has been addressed with methods and code
> such as that provided in [1].
> 
> Recent discussions about vmemmap freeing [2] have indicated that it
> would be beneficial to freeze all sub pages, including the head page
> of pages returned from low level allocators before converting to a
> hugetlb page.  This helps avoid races if we want to replace the page
> containing vmemmap for the head page.
> 
> There have been proposals to change at least the buddy allocator to
> return frozen pages as described at [3].  If such a change is made, it
> can be employed by the hugetlb code.  However, as mentioned above
> hugetlb uses several low level allocators so each would need to be
> modified to return frozen pages.  For now, we can manually freeze the
> returned pages.  This is done in two places:
> 1) alloc_buddy_huge_page, only the returned head page is ref counted.
>   We freeze the head page, retrying once in the VERY rare case where
>   there may be an inflated ref count.
> 2) prep_compound_gigantic_page, for gigantic pages the current code
>   freezes all pages except the head page.  New code will simply freeze
>   the head page as well.
> 
> In a few other places, code checks for inflated ref counts on newly
> allocated hugetlb pages.  With the modifications to freeze after
> allocating, this code can be removed.
> 
> After hugetlb pages are freshly allocated, they are often added to the
> hugetlb free lists.  Since these pages were previously ref counted, this
> was done via put_page() which would end up calling the hugetlb
> destructor: free_huge_page.  With changes to freeze pages, we simply
> call free_huge_page directly to add the pages to the free list.
> 
> In a few other places, freshly allocated hugetlb pages were immediately
> put into use, and the expectation was they were already ref counted.  In
> these cases, we must manually ref count the page.
> 
> [1] https://lore.kernel.org/linux-mm/20210622021423.154662-3-mike.kravetz@xxxxxxxxxx/
> [2] https://lore.kernel.org/linux-mm/20220802180309.19340-1-joao.m.martins@xxxxxxxxxx/
> [3] https://lore.kernel.org/linux-mm/20220809171854.3725722-1-willy@xxxxxxxxxxxxx/
> 
> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>

Thanks Mike.

Reviewed-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>