[PATCH v12 00/31] Speculative page faults

[Laurent Dufour]

Le 23/04/2019 à 11:38, Peter Zijlstra a écrit :
> On Mon, Apr 22, 2019 at 02:29:16PM -0700, Michel Lespinasse wrote:
>> The proposed spf mechanism only handles anon vmas. Is there a
>> fundamental reason why it couldn't handle mapped files too ?
>> My understanding is that the mechanism of verifying the vma after
>> taking back the ptl at the end of the fault would work there too ?
>> The file has to stay referenced during the fault, but holding the vma's
>> refcount could be made to cover that ? the vm_file refcount would have
>> to be released in __free_vma() instead of remove_vma; I'm not quite sure
>> if that has more implications than I realize ?
> 
> IIRC (and I really don't remember all that much) the trickiest bit was
> vs unmount. Since files can stay open past the 'expected' duration,
> umount could be delayed.
> 
> But yes, I think I had a version that did all that just 'fine'. Like
> mentioned, I didn't keep the refcount because it sucked just as hard as
> the mmap_sem contention, but the SRCU callback did the fput() just fine
> (esp. now that we have delayed_fput).

I had to use a refcount for the VMA because I'm using RCU in place of 
SRCU and only protecting the RB tree using RCU.

Regarding the file pointer, I decided to release it synchronously to 
avoid the latency of RCU during the file closing. As you mentioned this 
could delayed the umount but not only, as Linus Torvald demonstrated by 
the past [1]. Anyway, since the file support is not yet here there is no 
need for that currently.

Regarding the file mapping support, the concern is to ensure that 
vm_ops->fault() will not try to release the mmap_sem. This is true for 
most of the file system operation using the generic one, but there is 
currently no clever way to identify that except by checking the 
vm_ops->fault pointer. Adding a flag to the vm_operations_struct 
structure is another option.

that's doable as far as the underlying fault() function is not dealing 
with the mmap_sem, and I made a try by the past but was thinking that 
first the anonymous case should be accepted before moving forward this way.

[1] 
https://lore.kernel.org/linux-mm/alpine.LFD.2.00.1001041904250.3630@localhost.localdomain/