Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-version: 1.0
Content-transfer-encoding: 8BIT
Content-type: text/plain; charset="UTF-8"
Message-id: <5A715C2B.70609@samsung.com>
Date:   Wed, 31 Jan 2018 15:03:23 +0900
From:   Inki Dae <inki.dae@samsung.com>
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101
        Thunderbird/31.6.0
To:     Russell King - ARM Linux <linux@armlinux.org.uk>,
        Marek Szyprowski <m.szyprowski@samsung.com>
Cc:     linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Subject: Re: [RFC 1/2] arm: cacheflush syscall: process only pages that are
 in the memory
In-reply-to: <20180126213911.GY17719@n2100.armlinux.org.uk>
CMS-TYPE: 102P
DLP-Filter: Pass
References: <CGME20180126111453eucas1p1330d8561386c3cf2bb457bc22c0d99a8@eucas1p1.samsung.com>
        <20180126111441.29353-1-m.szyprowski@samsung.com>
        <20180126113226.GX17719@n2100.armlinux.org.uk>
        <589aa8ab-5bfc-e065-51f9-3a403c346d92@samsung.com>
        <20180126213911.GY17719@n2100.armlinux.org.uk>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Hi Russell,

2018년 01월 27일 06:39에 Russell King - ARM Linux 이(가) 쓴 글:
> On Fri, Jan 26, 2018 at 02:30:47PM +0100, Marek Szyprowski wrote:
>> Hi Russell,
>>
>> On 2018-01-26 12:32, Russell King - ARM Linux wrote:
>>> On Fri, Jan 26, 2018 at 12:14:40PM +0100, Marek Szyprowski wrote:
>>>> glibc in calls cacheflush syscall on the whole textrels section of the
>>>> relocated binaries. However, relocation usually doesn't touch all pages
>>>> of that section, so not all of them are read to memory when calling this
>>>> syscall. However flush_cache_user_range() function will unconditionally
>>>> touch all pages from the provided range, resulting additional overhead
>>>> related to reading all clean pages. Optimize this by calling
>>>> flush_cache_user_range() only on the pages that are already in the
>>>> memory.
>>> What ensures that another CPU doesn't remove a page while we're
>>> flushing it?  That will trigger a data abort, which will want to
>>> take the mmap_sem, causing a deadlock.
>>
>> I thought that taking mmap_sem will prevent pages from being removed.
>> mmap_sem has been already taken in the previous implementation of that
>> syscall, until code simplification done by commit 97c72d89ce0e ("ARM:
>> cacheflush: don't bother rounding to nearest vma").
> 
> No, you're not reading the previous code state correctly.  Take a closer
> look at that commit.
> 
> find_vma() requires that mmap_sem is held across the call as the VMA
> list is not stable without that semaphore held.  However, more
> importantly, notice that it drops the semaphore _before_ calling the
> cache flushing function (__do_cache_op()).
> 
> The point is that if __do_cache_op() faults, it will enter
> do_page_fault(), which will try to take the mmap_sem again, causing
> a deadlock.

I'm not sure but seems this patch tries to do cache-flush only in-memory pages.
So I think the page fault wouldn't happen becasue flush_cache_user_range function returns always 0.

Thanks,
Inki Dae

>