Re: [PATCH 2/4] MADV_VOLATILE: Add MADV_VOLATILE/NONVOLATILE hooks
and handle marking vmas
[Posted June 18, 2014 by corbet]
| From: |
| John Stultz <john.stultz-AT-linaro.org> |
| To: |
| Minchan Kim <minchan-AT-kernel.org> |
| Subject: |
| Re: [PATCH 2/4] MADV_VOLATILE: Add MADV_VOLATILE/NONVOLATILE hooks and handle marking vmas |
| Date: |
| Thu, 08 May 2014 09:38:40 -0700 |
| Message-ID: |
| <536BB310.1050105@linaro.org> |
| Cc: |
| LKML <linux-kernel-AT-vger.kernel.org>, Andrew Morton <akpm-AT-linux-foundation.org>, Android Kernel Team <kernel-team-AT-android.com>, Johannes Weiner <hannes-AT-cmpxchg.org>, Robert Love <rlove-AT-google.com>, Mel Gorman <mel-AT-csn.ul.ie>, Hugh Dickins <hughd-AT-google.com>, Dave Hansen <dave-AT-sr71.net>, Rik van Riel <riel-AT-redhat.com>, Dmitry Adamushko <dmitry.adamushko-AT-gmail.com>, Neil Brown <neilb-AT-suse.de>, Andrea Arcangeli <aarcange-AT-redhat.com>, Mike Hommey <mh-AT-glandium.org>, Taras Glek <tglek-AT-mozilla.com>, Jan Kara <jack-AT-suse.cz>, KOSAKI Motohiro <kosaki.motohiro-AT-gmail.com>, Michel Lespinasse <walken-AT-google.com>, Keith Packard <keithp-AT-keithp.com>, "linux-mm-AT-kvack.org" <linux-mm-AT-kvack.org> |
| Archive‑link: | |
Article |
On 05/07/2014 06:21 PM, Minchan Kim wrote:
> Hey John,
>
> On Tue, Apr 29, 2014 at 02:21:21PM -0700, John Stultz wrote:
>> This patch introduces MADV_VOLATILE/NONVOLATILE flags to madvise(),
>> which allows for specifying ranges of memory as volatile, and able
>> to be discarded by the system.
>>
>> This initial patch simply adds flag handling to madvise, and the
>> vma handling, splitting and merging the vmas as needed, and marking
>> them with VM_VOLATILE.
>>
>> No purging or discarding of volatile ranges is done at this point.
>>
>> This a simplified implementation which reuses some of the logic
>> from Minchan's earlier efforts. So credit to Minchan for his work.
> Remove purged argument is really good thing but I'm not sure merging
> the feature into madvise syscall is good idea.
> My concern is how we support user who don't want SIGBUS.
> I believe we should support them because someuser(ex, sanitizer) really
> want to avoid MADV_NONVOLATILE call right before overwriting their cache
> (ex, If there was purged page for cyclic cache, user should call NONVOLATILE
> right before overwriting to avoid SIGBUS).
So... Why not use MADV_FREE then for this case?
Just to be clear, by moving back to madvise, I'm not trying to replace
MADV_FREE. I think you're work there is still useful and splitting the
semantics between the two is cleaner.
> Moreover, this changes made unmarking cost O(N) so I'd like to avoid
> NOVOLATILE syscall if possible.
Well, I think that was made in v13, but yes. NONVOLATILE is currently an
expensive operation in order to keep the semantics simpler, as requested
by Johannes and Kosaki-san.
> For me, SIGBUS is more special usecase for code pages but I believe
> both are reasonable for each usecase so my preference is MADV_VOLATILE
> is just zero-filled page and MADV_VOLATILE_SIGBUS, another new advise
> if you really want to merge volatile range feature with madvise.
This I disagree with. Even for non-code page cases, SIGBUS on volatile
page access is important for normal users who might accidentally touch
volatile data, so they know they are corrupting their data. I know
Johannes suggested this is simply a use-after-free issue, but I really
feel it results in having very strange semantics. And for those cases
where there is a benefit to zero-fill, MADV_FREE seems more appropriate.
thanks
-john
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