Re: [PATCH 2/4] MADV_VOLATILE: Add MADV_VOLATILE/NONVOLATILE hooks and handle marking vmas

John Stultz
2014-05-08T16:38:47Z

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