Re: [PATCH 2 of 8] Change O_DIRECT to use placeholders instead of i_mutex/i_alloc_sem locking

[Zach Brown <zach.brown@oracle.com>]

>
> The test case Linus sent me boils down to this:
>
> fd = open(file)
> buffer = mmap(fd, 128 pages);
> close(fd);
> fd = open(file, O_DIRECT);
> write(fd, buffer, 66 pages);

Yeah, though I bet the inner close/open isn't needed.

> I think the deadlock is limited to cases where get_user_pages will get
> stuck in filemap_nopage waiting for placeholders inserted by this DIO.

Yeah.

> It looks like that can only happen when buffer is mapped at the  
> start of
> the dio.

At the *start* of the dio or by the time get_user_pages() is called?

The dio and mmap() aren't serialized, are they?  mmap() just sets up  
the vma, I thought, and will only touch the mmap_sem.

I'm fearing threads racing write(fd, buffer, ) and mmap(buffer,  
MAP_FIXED...).

I might just be missing the locking that serializes them.  If nothing  
else, this should be mentioned in the comment above the code that  
looks like a racy test against the presence of a mapping.

- z