Re: [PATCH v10 00/12] LUF(Lazy Unmap Flush) reducing tlb numbers over 90%

[Huang, Ying]

Byungchul Park <byungchul@xxxxxx> writes:

> On Fri, May 24, 2024 at 10:16:39AM -0700, Dave Hansen wrote:
>> On 5/9/24 23:51, Byungchul Park wrote:
>> > To achieve that:
>> > 
>> >    1. For the folios that map only to non-writable tlb entries, prevent
>> >       tlb flush during unmapping but perform it just before the folios
>> >       actually become used, out of buddy or pcp.
>> 
>> Is this just _pure_ unmapping (like MADV_DONTNEED), or does it apply to
>> changing the memory map, like munmap() itself?
>
> I think it can be applied to any unmapping of ro ones but LUF for now is
> working only with unmapping during folio migrion and reclaim.
>
>> >    2. When any non-writable ptes change to writable e.g. through fault
>> >       handler, give up luf mechanism and perform tlb flush required
>> >       right away.
>> > 
>> >    3. When a writable mapping is created e.g. through mmap(), give up
>> >       luf mechanism and perform tlb flush required right away.
>> 
>> Let's say you do this:
>> 
>> 	fd = open("/some/file", O_RDONLY);
>> 	ptr1 = mmap(-1, size, PROT_READ, ..., fd, ...);
>> 	foo1 = *ptr1;
>> 
>> You now have a read-only PTE pointing to the first page of /some/file.
>> Let's say try_to_unmap() comes along and decides it can_luf_folio().
>> The page gets pulled out of the page cache and freed, the PTE is zeroed.
>>  But the TLB is never flushed.
>> 
>> Now, someone does:
>> 
>> 	fd2 = open("/some/other/file", O_RDONLY);
>> 	ptr2 = mmap(ptr1, size, PROT_READ, MAP_FIXED, fd, ...);
>> 	foo2 = *ptr2;
>> 
>> and they overwrite the old VMA.  Does foo2 have the contents of the new
>> "/some/other/file" or the old "/some/file"?  How does the new mmap()
>
> Good point.  It should've give up LUF at the 2nd mmap() in this case.
> I will fix it by introducing a new flag in task_struct indicating if LUF
> has left stale maps for the task so that LUF can give up and flush right
> away in mmap().
>
>> know that there was something to flush?
>> 
>> BTW, the same thing could happen without a new mmap().  Someone could
>> modify the file in the middle, maybe even from another process.
>
> Thank you for the pointing out.  I will fix it too by introducing a new
> flag in inode or something to make LUF aware if updating the file has
> been tried so that LUF can give up and flush right away in the case.
>
> Plus, I will add another give-up at code changing the permission of vma
> to writable.

I guess that you need a framework similar as
"flush_tlb_batched_pending()" to deal with interaction with other TLB
related operations.

--
Best Regards,
Huang, Ying

> Thank you very much.
>
> 	Byungchul
>
>> 	fd = open("/some/file", O_RDONLY);
>> 	ptr1 = mmap(-1, size, PROT_READ, ..., fd, ...);
>> 	foo1 = *ptr1;
>> 	// LUF happens here
>> 	// "/some/file" changes
>> 	foo2 = *ptr1; // Does this see the change?