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Shutemov" To: Ralph Campbell Cc: akpm@linux-foundation.org, Andrea Arcangeli , Zi Yan , Yang Shi , linux-mm@kvack.org, linux-kernel@vger.kernel.org, "Kirill A. Shutemov" Subject: Re: [PATCHv2 2/8] khugepaged: Do not stop collapse if less than half PTEs are referenced Message-ID: <20200409133414.hcynyxggt24nitd5@box> References: <20200403112928.19742-1-kirill.shutemov@linux.intel.com> <20200403112928.19742-3-kirill.shutemov@linux.intel.com> <7d3af6a4-b0ee-9497-da0b-b93e050c5a4c@nvidia.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <7d3af6a4-b0ee-9497-da0b-b93e050c5a4c@nvidia.com> X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, Apr 06, 2020 at 12:53:00PM -0700, Ralph Campbell wrote: > > On 4/3/20 4:29 AM, Kirill A. Shutemov wrote: > > __collapse_huge_page_swapin() check number of referenced PTE to decide > > if the memory range is hot enough to justify swapin. > > > > The problem is that it stops collapse altogether if there's not enough > > referenced pages, not only swappingin. > > > > Signed-off-by: Kirill A. Shutemov > > Fixes: 0db501f7a34c ("mm, thp: convert from optimistic swapin collapsing to conservative") > > Reviewed-by: Zi Yan --- > > mm/khugepaged.c | 3 ++- > > 1 file changed, 2 insertions(+), 1 deletion(-) > > > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > > index 99bab7e4d05b..14d7afc90786 100644 > > --- a/mm/khugepaged.c > > +++ b/mm/khugepaged.c > > @@ -905,7 +905,8 @@ static bool __collapse_huge_page_swapin(struct mm_struct *mm, > > /* we only decide to swapin, if there is enough young ptes */ > > if (referenced < HPAGE_PMD_NR/2) { > > trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0); > > The trace point is recording the return value. Shouldn't you s/0/1 to match? Good catch. > > > - return false; > > + /* Do not block collapse, only skip swapping in */ > > + return true; > > } > > vmf.pte = pte_offset_map(pmd, address); > > for (; vmf.address < address + HPAGE_PMD_NR*PAGE_SIZE; > > > > So "referenced < HPAGE_PMD_NR/2" means swapped out pages aren't faulted back in > but there could still be no swapped out pages, just "old" pages so collapse could > succeed. Seems like this check could have been made in khugepaged_scan_pmd() when > "referenced" is being computed and "unmapped" is available. Just skip setting > "ret = 1" if unmapped > 0 && referenced < HPAGE_PMD_NR/2. Good point. I'll change the code to address it. -- Kirill A. Shutemov