From: Vineet Gupta <Vineet.Gupta1@synopsys.com>
To: <linux-snps-arc@lists.infradead.org>
Cc: Peter Zijlstra <peterz@infradead.org>, <paltsev@snyopsys.com>,
<linux-kernel@vger.kernel.org>,
"linux-mm@kvack.org" <linux-mm@kvack.org>
Subject: Re: [PATCH 9/9] ARC: mm: do_page_fault refactor #8: release mmap_sem sooner
Date: Thu, 30 May 2019 09:48:34 -0700 [thread overview]
Message-ID: <d7284369-52a6-71e5-65ae-75ec389eb92b@synopsys.com> (raw)
In-Reply-To: <1557880176-24964-10-git-send-email-vgupta@synopsys.com>
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 <peterz@infradead.org>
> Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
> ---
> 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;
>
>
prev parent reply other threads:[~2019-05-30 16:48 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-15 0:29 [PATCH 0/9] ARC do_page_fault rework Vineet Gupta
2019-05-15 0:29 ` [PATCH 1/9] ARC: mm: SIGSEGV userspace trying to access kernel virtual memory Vineet Gupta
2019-05-15 0:29 ` [PATCH 2/9] ARC: mm: do_page_fault refactor #1: remove label @good_area Vineet Gupta
2019-05-15 0:29 ` [PATCH 3/9] ARC: mm: do_page_fault refactor #2: remove short lived variable Vineet Gupta
2019-05-15 0:29 ` [PATCH 4/9] ARC: mm: do_page_fault refactor #3: tidyup vma access permission code Vineet Gupta
2019-05-16 17:24 ` Eugeniy Paltsev
2019-05-16 17:37 ` Vineet Gupta
2019-05-16 17:44 ` Alexey Brodkin
2019-05-16 18:57 ` Vineet Gupta
2019-05-17 22:23 ` Eugeniy Paltsev
2019-05-30 17:58 ` extraneous generated EXTB (was Re: [PATCH 4/9] ARC: mm: do_page_fault refactor #3: tidyup vma access permission code) Vineet Gupta
2019-05-15 0:29 ` [PATCH 5/9] ARC: mm: do_page_fault refactor #4: consolidate retry related logic Vineet Gupta
2019-05-15 0:29 ` [PATCH 6/9] ARC: mm: do_page_fault refactor #5: scoot no_context to end Vineet Gupta
2019-05-15 0:29 ` [PATCH 7/9] ARC: mm: do_page_fault refactor #6: error handlers to use same pattern Vineet Gupta
2019-05-15 0:29 ` [PATCH 8/9] ARC: mm: do_page_fault refactor #7: fold the various error handling Vineet Gupta
2019-05-15 0:29 ` [PATCH 9/9] ARC: mm: do_page_fault refactor #8: release mmap_sem sooner Vineet Gupta
2019-05-30 16:48 ` Vineet Gupta [this message]
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=d7284369-52a6-71e5-65ae-75ec389eb92b@synopsys.com \
--to=vineet.gupta1@synopsys.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=linux-snps-arc@lists.infradead.org \
--cc=paltsev@snyopsys.com \
--cc=peterz@infradead.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).