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Thu, 30 May 2019 16:48:49 +0000 (UTC) Received: from IN01WEHTCB.internal.synopsys.com (10.144.199.106) by us01wehtc1.internal.synopsys.com (10.12.239.231) with Microsoft SMTP Server (TLS) id 14.3.408.0; Thu, 30 May 2019 09:48:45 -0700 Received: from IN01WEHTCA.internal.synopsys.com (10.144.199.103) by IN01WEHTCB.internal.synopsys.com (10.144.199.105) with Microsoft SMTP Server (TLS) id 14.3.408.0; Thu, 30 May 2019 22:18:42 +0530 Received: from [10.10.161.35] (10.10.161.35) by IN01WEHTCA.internal.synopsys.com (10.144.199.243) with Microsoft SMTP Server (TLS) id 14.3.408.0; Thu, 30 May 2019 22:18:54 +0530 Subject: Re: [PATCH 9/9] ARC: mm: do_page_fault refactor #8: release mmap_sem sooner To: CC: Peter Zijlstra , , , "linux-mm@kvack.org" Newsgroups: gmane.linux.kernel.arc,gmane.linux.kernel References: <1557880176-24964-1-git-send-email-vgupta@synopsys.com> <1557880176-24964-10-git-send-email-vgupta@synopsys.com> From: Vineet Gupta Openpgp: preference=signencrypt Autocrypt: addr=vgupta@synopsys.com; keydata= mQINBFEffBMBEADIXSn0fEQcM8GPYFZyvBrY8456hGplRnLLFimPi/BBGFA24IR+B/Vh/EFk B5LAyKuPEEbR3WSVB1x7TovwEErPWKmhHFbyugdCKDv7qWVj7pOB+vqycTG3i16eixB69row lDkZ2RQyy1i/wOtHt8Kr69V9aMOIVIlBNjx5vNOjxfOLux3C0SRl1veA8sdkoSACY3McOqJ8 zR8q1mZDRHCfz+aNxgmVIVFN2JY29zBNOeCzNL1b6ndjU73whH/1hd9YMx2Sp149T8MBpkuQ cFYUPYm8Mn0dQ5PHAide+D3iKCHMupX0ux1Y6g7Ym9jhVtxq3OdUI5I5vsED7NgV9c8++baM 7j7ext5v0l8UeulHfj4LglTaJIvwbUrCGgtyS9haKlUHbmey/af1j0sTrGxZs1ky1cTX7yeF nSYs12GRiVZkh/Pf3nRLkjV+kH++ZtR1GZLqwamiYZhAHjo1Vzyl50JT9EuX07/XTyq/Bx6E dcJWr79ZphJ+mR2HrMdvZo3VSpXEgjROpYlD4GKUApFxW6RrZkvMzuR2bqi48FThXKhFXJBd JiTfiO8tpXaHg/yh/V9vNQqdu7KmZIuZ0EdeZHoXe+8lxoNyQPcPSj7LcmE6gONJR8ZqAzyk F5voeRIy005ZmJJ3VOH3Gw6Gz49LVy7Kz72yo1IPHZJNpSV5xwARAQABtCpWaW5lZXQgR3Vw dGEgKGFsaWFzKSA8dmd1cHRhQHN5bm9wc3lzLmNvbT6JAj4EEwECACgCGwMGCwkIBwMCBhUI AgkKCwQWAgMBAh4BAheABQJbBYpwBQkLx0HcAAoJEGnX8d3iisJeChAQAMR2UVbJyydOv3aV jmqP47gVFq4Qml1weP5z6czl1I8n37bIhdW0/lV2Zll+yU1YGpMgdDTHiDqnGWi4pJeu4+c5 xsI/VqkH6WWXpfruhDsbJ3IJQ46//jb79ogjm6VVeGlOOYxx/G/RUUXZ12+CMPQo7Bv+Jb+t NJnYXYMND2Dlr2TiRahFeeQo8uFbeEdJGDsSIbkOV0jzrYUAPeBwdN8N0eOB19KUgPqPAC4W HCg2LJ/o6/BImN7bhEFDFu7gTT0nqFVZNXlOw4UcGGpM3dq/qu8ZgRE0turY9SsjKsJYKvg4 djAaOh7H9NJK72JOjUhXY/sMBwW5vnNwFyXCB5t4ZcNxStoxrMtyf35synJVinFy6wCzH3eJ XYNfFsv4gjF3l9VYmGEJeI8JG/ljYQVjsQxcrU1lf8lfARuNkleUL8Y3rtxn6eZVtAlJE8q2 hBgu/RUj79BKnWEPFmxfKsaj8of+5wubTkP0I5tXh0akKZlVwQ3lbDdHxznejcVCwyjXBSny d0+qKIXX1eMh0/5sDYM06/B34rQyq9HZVVPRHdvsfwCU0s3G+5Fai02mK68okr8TECOzqZtG cuQmkAeegdY70Bpzfbwxo45WWQq8dSRURA7KDeY5LutMphQPIP2syqgIaiEatHgwetyVCOt6 tf3ClCidHNaGky9KcNSQ Message-ID: Date: Thu, 30 May 2019 09:48:34 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.0 MIME-Version: 1.0 In-Reply-To: <1557880176-24964-10-git-send-email-vgupta@synopsys.com> Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.10.161.35] Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 5/14/19 5:29 PM, Vineet Gupta wrote: > In case of successful page fault handling, this patch releases mmap_sem > before updating the perf stat event for major/minor faults. So even > though the contention reduction is NOT supe rhigh, it is still an > improvement. This patch causes regression: LMBench lat_sig triggers a bogus oom. The issue is not due to code movement of up_read() but as artifact of @fault initialized to VM_FAULT_ERROR. The lat_sig invalid access doesn't take handle_mm_fault() instead it hits !VM_WRITE case for write access leading to use of "initialized" value of @fault VM_FAULT_ERROR which includes VM_FAULT_OOM hence the bogus oom handling. > There's an additional code size improvement as we only have 2 up_read() > calls now. > > Note to myself: > -------------- > > 1. Given the way it is done, we are forced to move @bad_area label earlier > causing the various "goto bad_area" cases to hit perf stat code. > > - PERF_COUNT_SW_PAGE_FAULTS is NOW updated for access errors which is what > arm/arm64 seem to be doing as well (with slightly different code) > - PERF_COUNT_SW_PAGE_FAULTS_{MAJ,MIN} must NOT be updated for the > error case which is guarded by now setting @fault initial value > to VM_FAULT_ERROR which serves both cases when handle_mm_fault() > returns error or is not called at all. > > 2. arm/arm64 use two homebrew fault flags VM_FAULT_BAD{MAP,MAPACCESS} > which I was inclined to add too but seems not needed for ARC > > - given that we have everything is 1 function we cabn stil use goto > - we setup si_code at the right place (arm* do that in the end) > - we init fault already to error value which guards entry into perf > stats event update > > Cc: Peter Zijlstra > Signed-off-by: Vineet Gupta > --- > arch/arc/mm/fault.c | 9 ++++----- > 1 file changed, 4 insertions(+), 5 deletions(-) > > diff --git a/arch/arc/mm/fault.c b/arch/arc/mm/fault.c > index 20402217d9da..e93ea06c214c 100644 > --- a/arch/arc/mm/fault.c > +++ b/arch/arc/mm/fault.c > @@ -68,7 +68,7 @@ void do_page_fault(unsigned long address, struct pt_regs *regs) > struct mm_struct *mm = tsk->mm; > int sig, si_code = SEGV_MAPERR; > unsigned int write = 0, exec = 0, mask; > - vm_fault_t fault; /* handle_mm_fault() output */ > + vm_fault_t fault = VM_FAULT_ERROR; /* handle_mm_fault() output */ > unsigned int flags; /* handle_mm_fault() input */ > > /* > @@ -155,6 +155,9 @@ void do_page_fault(unsigned long address, struct pt_regs *regs) > } > } > > +bad_area: > + up_read(&mm->mmap_sem); > + > /* > * Major/minor page fault accounting > * (in case of retry we only land here once) > @@ -173,13 +176,9 @@ void do_page_fault(unsigned long address, struct pt_regs *regs) > } > > /* Normal return path: fault Handled Gracefully */ > - up_read(&mm->mmap_sem); > return; > } > > -bad_area: > - up_read(&mm->mmap_sem); > - > if (!user_mode(regs)) > goto no_context; > >