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SubjectRe: [PATCH 5/7] khugepaged: Allow to collapse PTE-mapped compound pages
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'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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