Messages in this thread | | | Date | Mon, 23 Jun 2014 13:39:29 +0800 | From | Zhang Yanfei <> | Subject | Re: [PATCH v3 01/13] mm, THP: don't hold mmap_sem in khugepaged when allocating THP |
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Hello
On 06/21/2014 01:45 AM, Kirill A. Shutemov wrote: > On Fri, Jun 20, 2014 at 05:49:31PM +0200, Vlastimil Babka wrote: >> When allocating huge page for collapsing, khugepaged currently holds mmap_sem >> for reading on the mm where collapsing occurs. Afterwards the read lock is >> dropped before write lock is taken on the same mmap_sem. >> >> Holding mmap_sem during whole huge page allocation is therefore useless, the >> vma needs to be rechecked after taking the write lock anyway. Furthemore, huge >> page allocation might involve a rather long sync compaction, and thus block >> any mmap_sem writers and i.e. affect workloads that perform frequent m(un)map >> or mprotect oterations. >> >> This patch simply releases the read lock before allocating a huge page. It >> also deletes an outdated comment that assumed vma must be stable, as it was >> using alloc_hugepage_vma(). This is no longer true since commit 9f1b868a13 >> ("mm: thp: khugepaged: add policy for finding target node"). > > There is no point in touching ->mmap_sem in khugepaged_alloc_page() at > all. Please, move up_read() outside khugepaged_alloc_page(). >
I might be wrong. If we up_read in khugepaged_scan_pmd(), then if we round again do the for loop to get the next vma and handle it. Does we do this without holding the mmap_sem in any mode?
And if the loop end, we have another up_read in breakouterloop. What if we have released the mmap_sem in collapse_huge_page()?
-- Thanks. Zhang Yanfei
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