Re: [PATCH v3 01/17] mm: support madvise(MADV_FREE)

[yalin wang]

> On Nov 16, 2015, at 10:13, Minchan Kim <minchan@xxxxxxxxxx> wrote:
> 
> On Fri, Nov 13, 2015 at 11:46:07AM -0800, Andy Lutomirski wrote:
>> On Fri, Nov 13, 2015 at 12:13 AM, Daniel Micay <danielmicay@xxxxxxxxx> wrote:
>>> On 13/11/15 02:03 AM, Minchan Kim wrote:
>>>> On Fri, Nov 13, 2015 at 01:45:52AM -0500, Daniel Micay wrote:
>>>>>> And now I am thinking if we use access bit, we could implment MADV_FREE_UNDO
>>>>>> easily when we need it. Maybe, that's what you want. Right?
>>>>> 
>>>>> Yes, but why the access bit instead of the dirty bit for that? It could
>>>>> always be made more strict (i.e. access bit) in the future, while going
>>>>> the other way won't be possible. So I think the dirty bit is really the
>>>>> more conservative choice since if it turns out to be a mistake it can be
>>>>> fixed without a backwards incompatible change.
>>>> 
>>>> Absolutely true. That's why I insist on dirty bit until now although
>>>> I didn't tell the reason. But I thought you wanted to change for using
>>>> access bit for the future, too. It seems MADV_FREE start to bloat
>>>> over and over again before knowing real problems and usecases.
>>>> It's almost same situation with volatile ranges so I really want to
>>>> stop at proper point which maintainer should decide, I hope.
>>>> Without it, we will make the feature a lot heavy by just brain storming
>>>> and then causes lots of churn in MM code without real bebenfit
>>>> It would be very painful for us.
>>> 
>>> Well, I don't think you need more than a good API and an implementation
>>> with no known bugs, kernel security concerns or backwards compatibility
>>> issues. Configuration and API extensions are something for later (i.e.
>>> land a baseline, then submit stuff like sysctl tunables). Just my take
>>> on it though...
>>> 
>> 
>> As long as it's anonymous MAP_PRIVATE only, then the security aspects
>> should be okay.  MADV_DONTNEED seems to work on pretty much any VMA,
>> and there's been long history of interesting bugs there.
>> 
>> As for dirty vs accessed, an argument in favor of going straight to
>> accessed is that it means that users can write code like this without
>> worrying about whether they have a kernel that uses the dirty bit:
>> 
>> x = mmap(...);
>> *x = 1;  /* mark it present */
>> 
>> /* i'm done with it */
>> *x = 1;
>> madvise(MADV_FREE, x, ...);
>> 
>> wait a while;
>> 
>> /* is it still there? */
>> if (*x == 1) {
>>  /* use whatever was cached there */
>> } else {
>> /* reinitialize it */
>> *x = 1;
>> }
>> 
>> With the dirty bit, this will look like it works, but on occasion
>> users will lose the race where they probe *x to see if the data was
>> lost and then the data gets lost before the next write comes in.
>> 
>> Sure, that load from *x could be changed to RMW or users could do a
>> dummy write (e.g. x[1] = 1; if (*x == 1) ...), but people might forget
>> to do that, and the caching implications are a little bit worse.
> 
> I think your example is the case what people abuse MADV_FREE.
> What happens if the object(ie, x) spans multiple pages?
> User should know object's memory align and investigate all of pages
> which span the object. Hmm, I don't think it's good for API.
> 
>> 
>> Note that switching to RMW is really really dangerous.  Doing:
>> 
>> *x &= 1;
>> if (*x == 1) ...;
>> 
>> is safe on x86 if the compiler generates:
>> 
>> andl $1, (%[x]);
>> cmpl $1, (%[x]);
>> 
>> but is unsafe if the compiler generates:
>> 
>> movl (%[x]), %eax;
>> andl $1, %eax;
>> movl %eax, (%[x]);
>> cmpl $1, %eax;
>> 
>> and even worse if the write is omitted when "provably" unnecessary.
>> 
>> OTOH, if switching to the accessed bit is too much of a mess, then
>> using the dirty bit at first isn't so bad.
> 
> Thanks! I want to use dirty bit first.
> 
> About access bit, I don't want to say it to mess but I guess it would
> change a lot subtle thing for all architectures. Because we have used
> access bit as just *hint* for aging while dirty bit is really
> *critical marker* for system integrity. A example in x86, we don't
> keep accuracy of access bit for reducing TLB flush IPI. I don't know
> what technique other arches have used but they might have.
> 
> Thanks.
> 
i think use access bit is not easy to implement for ANON page in kernel.
we are sure the Anon page is always PageDirty()  if it is !PageSwapCache() ,
unless it is MADV_FREE page ,
but use access bit , how to distinguish Normal ANON page and  MADV_FREE page?
it can be implemented by Access bit , but not easy, need more code change .

Thanks



 




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