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    SubjectRe: [External] Re: [RFC PATCH 00/24] mm/hugetlb: Free some vmemmap pages of hugetlb page
    On Wed, Sep 16, 2020 at 01:32:46AM +0800, Muchun Song wrote:
    > On Tue, Sep 15, 2020 at 11:42 PM Matthew Wilcox <willy@infradead.org> wrote:
    > >
    > > On Tue, Sep 15, 2020 at 11:28:01PM +0800, Muchun Song wrote:
    > > > On Tue, Sep 15, 2020 at 10:32 PM Matthew Wilcox <willy@infradead.org> wrote:
    > > > >
    > > > > On Tue, Sep 15, 2020 at 08:59:23PM +0800, Muchun Song wrote:
    > > > > > This patch series will free some vmemmap pages(struct page structures)
    > > > > > associated with each hugetlbpage when preallocated to save memory.
    > > > >
    > > > > It would be lovely to be able to do this. Unfortunately, it's completely
    > > > > impossible right now. Consider, for example, get_user_pages() called
    > > > > on the fifth page of a hugetlb page.
    > > >
    > > > Can you elaborate on the problem? Thanks so much.
    > >
    > > OK, let's say you want to do a 2kB I/O to offset 0x5000 of a 2MB page
    > > on a 4kB base page system. Today, that results in a bio_vec containing
    > > {head+5, 0, 0x800}. Then we call page_to_phys() on that (head+5) struct
    > > page to get the physical address of the I/O, and we turn it into a struct
    > > scatterlist, which similarly has a reference to the page (head+5).
    >
    > As I know, in this case, the get_user_pages() will get a reference
    > to the head page (head+0) before returning such that the hugetlb
    > page can not be freed. Although get_user_pages() returns the
    > page (head+5) and the scatterlist has a reference to the page
    > (head+5), this patch series can handle this situation. I can not
    > figure out where the problem is. What I missed? Thanks.

    You freed pages 4-511 from the vmemmap so they could be used for
    something else. Page 5 isn't there any more. So if you return head+5,
    then when we complete the I/O, we'll look for the compound_head() of
    head+5 and we won't find head.

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