Messages in this thread | | | Date | Fri, 11 Jun 2021 16:25:10 +0000 | From | Al Viro <> | Subject | Re: [RFC 4/9] gfs2: Fix mmap + page fault deadlocks (part 1) |
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On Wed, Jun 02, 2021 at 01:16:32PM +0200, Andreas Gruenbacher wrote:
> Well, iomap_file_buffered_write() does that by using > iov_iter_fault_in_readable() and iov_iter_copy_from_user_atomic() as > in iomap_write_actor(), but the read and direct I/O side doesn't seem > to have equivalents. I suspect we can't just wrap > generic_file_read_iter() and iomap_dio_rw() calls in > pagefault_disable().
And it will have zero effect on O_DIRECT case, so you get the same deadlocks right back. Because there you hit iomap_dio_bio_actor() bio_iov_iter_get_pages() .... get_user_pages_fast() .... faultin_page() handle_mm_fault() and at no point had CPU hit an exception, so disable_pagefault() will have no effect whatsoever. You can bloody well hit gfs2 readpage/mkwrite if the destination is in mmapped area of some GFS2 file. Do that while holding GFS2 locks and you are fucked.
No amount of prefaulting will protect you, BTW - it might make the deadlock harder to reproduce, but that's it.
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