linux-kernel - Re: [PATCH v8 1/2] hugetlb: don't delete vma_lock in hugetlb MADV

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Date:   Thu, 10 Nov 2022 14:22:33 -0800
From:   Nadav Amit <nadav.amit@...il.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Linux-MM <linux-mm@...ck.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        David Hildenbrand <david@...hat.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Peter Xu <peterx@...hat.com>, Rik van Riel <riel@...riel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Wei Chen <harperchen1110@...il.com>, stable@...r.kernel.org
Subject: Re: [PATCH v8 1/2] hugetlb: don't delete vma_lock in hugetlb
 MADV_DONTNEED processing

On Nov 10, 2022, at 1:48 PM, Mike Kravetz <mike.kravetz@...cle.com> wrote:

>>> void unmap_hugepage_range(struct vm_area_struct *vma, unsigned long start,
>>> 			  unsigned long end, struct page *ref_page,
>>> 			  zap_flags_t zap_flags)
>>> {
>>> +	struct mmu_notifier_range range;
>>> 	struct mmu_gather tlb;
>>> 
>>> +	mmu_notifier_range_init(&range, MMU_NOTIFY_UNMAP, 0, vma, vma->vm_mm,
>>> +				start, end);
>>> +	adjust_range_if_pmd_sharing_possible(vma, &range.start, &range.end);
>>> 	tlb_gather_mmu(&tlb, vma->vm_mm);
>>> +
>>> 	__unmap_hugepage_range(&tlb, vma, start, end, ref_page, zap_flags);
>> 
>> Is there a reason for not using range.start and range.end?
> 
> After calling adjust_range_if_pmd_sharing_possible, range.start - range.end
> could be much greater than the range we actually want to unmap.  The range
> gets adjusted to account for pmd sharing if that is POSSIBLE.  It does not
> know for sure if we will actually 'unshare a pmd'.
> 
> I suppose adjust_range_if_pmd_sharing_possible could be modified to actually
> check if unmapping will result in unsharing, but it does not do that today.

Thanks for the explanation. It’s probably me, but I am still not sure that I
understand the the different between __unmap_hugepage_range() using (start,
end) and __zap_page_range_single() using (address, range.end). Perhaps it
worth a comment in the code?

But anyhow… shouldn’t unmap_hugepage_range() call
mmu_notifier_invalidate_range_start()?