From: Al Viro <viro@zeniv.linux.org.uk>
To: afzal mohammed <afzal.mohd.ma@gmail.com>
Cc: Arnd Bergmann <arnd@arndb.de>,
Russell King - ARM Linux admin <linux@armlinux.org.uk>,
Linus Walleij <linus.walleij@linaro.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Linux-MM <linux-mm@kvack.org>,
Linux ARM <linux-arm-kernel@lists.infradead.org>,
Nicolas Pitre <nico@fluxnic.net>,
Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will@kernel.org>
Subject: Re: [RFC 1/3] lib: copy_{from,to}_user using gup & kmap_atomic()
Date: Sat, 13 Jun 2020 13:56:15 +0100 [thread overview]
Message-ID: <20200613125615.GF23230@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20200613125126.GE23230@ZenIV.linux.org.uk>
On Sat, Jun 13, 2020 at 01:51:26PM +0100, Al Viro wrote:
> On Sat, Jun 13, 2020 at 05:34:32PM +0530, afzal mohammed wrote:
>
> > Observation is that max. pages reaching copy_{from,to}_user() is 2,
> > observed maximum of n (number of bytes) being 1 page size. i think C
> > library cuts any size read, write to page size (if it exceeds) &
> > invokes the system call. Max. pages reaching 2, happens when 'n'
> > crosses page boundary, this has been observed w/ small size request
> > as well w/ ones of exact page size (but not page aligned).
> >
> > Even w/ dd of various size >4K, never is the number of pages required
> > to be mapped going greater than 2 (even w/ 'dd' 'bs=1M')
> >
> > i have a worry (don't know whether it is an unnecessary one): even
> > if we improve performance w/ large copy sizes, it might end up in a
> > sluggishness w.r.t user experience due to most (hence a high amount)
> > of user copy calls being few bytes & there the penalty being higher.
> > And benchmark would not be able to detect anything abnormal since
> > usercopy are being tested on large sizes.
> >
> > Quickly comparing boot-time on Beagle Bone White, boot time increases
> > by only 4%, perhaps this worry is irrelevant, but just thought will
> > put it across.
>
> Do stat(2) of the same tmpfs file in a loop (on tmpfs, to eliminate
> the filesystem playing silly buggers). And I wouldn't expect anything
> good there...
Incidentally, what about get_user()/put_user()? _That_ is where it's
going to really hurt...
next prev parent reply other threads:[~2020-06-13 12:56 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-12 10:17 [RFC 0/3] ARM: copy_{from,to}_user() for vmsplit 4g/4g afzal mohammed
2020-06-12 10:17 ` [RFC 1/3] lib: copy_{from,to}_user using gup & kmap_atomic() afzal mohammed
2020-06-12 12:02 ` Arnd Bergmann
2020-06-12 13:55 ` afzal mohammed
2020-06-12 20:07 ` Arnd Bergmann
2020-06-13 12:04 ` afzal mohammed
2020-06-13 12:51 ` Al Viro
2020-06-13 12:56 ` Al Viro [this message]
2020-06-13 13:42 ` afzal mohammed
2020-06-13 15:31 ` Al Viro
2020-06-13 15:41 ` Al Viro
2020-06-13 16:00 ` Al Viro
2020-06-13 18:55 ` Arnd Bergmann
2020-06-15 11:22 ` David Laight
2020-06-13 13:15 ` Russell King - ARM Linux admin
2020-06-14 13:06 ` afzal mohammed
2020-06-13 20:45 ` Arnd Bergmann
2020-06-13 22:16 ` Matthew Wilcox
2020-06-14 13:21 ` afzal mohammed
2020-06-14 14:55 ` afzal mohammed
2020-06-13 11:08 ` Andy Shevchenko
2020-06-13 13:29 ` afzal mohammed
2020-06-12 10:18 ` [RFC 2/3] ARM: uaccess: let UACCESS_GUP_KMAP_MEMCPY enabling afzal mohammed
2020-06-12 10:18 ` [RFC 3/3] ARM: provide CONFIG_VMSPLIT_4G_DEV for development afzal mohammed
2020-06-12 15:19 ` [RFC 0/3] ARM: copy_{from,to}_user() for vmsplit 4g/4g Nicolas Pitre
2020-06-12 16:01 ` afzal mohammed
2020-06-12 16:03 ` afzal mohammed
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