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    SubjectRe: [PATCH 5/7] khugepaged: Allow to collapse PTE-mapped compound pages
    On Sat, Mar 28, 2020 at 5:33 AM Kirill A. Shutemov <kirill@shutemov.name> wrote:
    >
    > On Fri, Mar 27, 2020 at 09:17:00PM -0400, Zi Yan wrote:
    > > > The compound page may be locked here if the function called for the first
    > > > time for the page and not locked after that (becouse we've unlocked it we
    > > > saw it the first time). The same with LRU.
    > > >
    > >
    > > For the first time, the compound page is locked and not on LRU, so this VM_BUG_ON passes.
    > > For the second time and so on, the compound page is unlocked and on the LRU,
    > > so this VM_BUG_ON still passes.
    > >
    > > For base page, VM_BUG_ON passes.
    > >
    > > Other unexpected situation (a compound page is locked and on LRU) triggers the VM_BU_ON,
    > > but your VM_BUG_ON will not detect this situation, right?
    >
    > Right. I will rework this code. I've just realized it is racy: after
    > unlock and putback on LRU the page can be locked by somebody else and this
    > code can unlock it which completely borken.

    I'm wondering if we shall skip putting the page back to LRU. Since the
    page is about to be freed soon, so as I mentioned in the other patch
    it sounds not very productive to put it back to LRU again.

    >
    > I'll pass down compound_pagelist to release_pte_pages() and handle the
    > situation there.
    >
    > > >>> if (likely(writable)) {
    > > >>> if (likely(referenced)) {
    > > >>
    > > >> Do we need a list here? There should be at most one compound page we will see here, right?
    > > >
    > > > Um? It's outside the pte loop. We get here once per PMD range.
    > > >
    > > > 'page' argument to trace_mm_collapse_huge_page_isolate() is misleading:
    > > > it's just the last page handled in the loop.
    > > >
    > >
    > > Throughout the pte loop, we should only see at most one compound page, right?
    >
    > No. mremap(2) opens a possibility for HPAGE_PMD_NR compound pages for
    > single PMD range.
    >
    >
    > --
    > Kirill A. Shutemov
    >

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