Re: [PATCH v3 01/13] mm, THP: don't hold mmap_sem in khugepaged when allocating THP

[Kirill A. Shutemov]

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 also deletes an outdated comment that assumed vma must be stable, as it was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().

-- 
 Kirill A. Shutemov
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