lkml.org 
[lkml]   [2021]   [Oct]   [26]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH v8 00/17] gfs2: Fix mmap + page fault deadlocks
    On Mon, Oct 25, 2021 at 09:00:43PM +0200, Andreas Gruenbacher wrote:
    > On Fri, Oct 22, 2021 at 9:23 PM Linus Torvalds
    > <torvalds@linux-foundation.org> wrote:
    > > On Fri, Oct 22, 2021 at 8:06 AM Catalin Marinas <catalin.marinas@arm.com> wrote:
    > > > Probing only the first byte(s) in fault_in() would be ideal, no need to
    > > > go through all filesystems and try to change the uaccess/probing order.
    > >
    > > Let's try that. Or rather: probing just the first page - since there
    > > are users like that btrfs ioctl, and the direct-io path.
    >
    > For direct I/O, we actually only want to trigger page fault-in so that
    > we can grab page references with bio_iov_iter_get_pages. Probing for
    > sub-page error domains will only slow things down. If we hit -EFAULT
    > during the actual copy-in or copy-out, we know that the error can't be
    > page fault related. Similarly, in the buffered I/O case, we only
    > really care about the next byte, so any probing beyond that is
    > unnecessary.
    >
    > So maybe we should split the sub-page error domain probing off from
    > the fault-in functions. Or at least add an argument to the fault-in
    > functions that specifies the amount of memory to probe.

    My preferred option is not to touch fault-in for sub-page faults (though
    I have some draft patches, they need testing).

    All this fault-in and uaccess with pagefaults_disabled() is needed to
    avoid a deadlock when the uaccess fault handling would take the same
    lock. With sub-page faults, the kernel cannot fix it up anyway, so the
    arch code won't even attempt call handle_mm_fault() (it is not an mm
    fault). But the problem is the copy_*_user() etc. API that can only
    return the number of bytes not copied. That's what I think should be
    fixed. fault_in() feels like the wrong place to address this when it's
    not an mm fault.

    As for fault_in() getting another argument with the amount of sub-page
    probing to do, I think the API gets even more confusing. I was also
    thinking, with your patches for fault_in() now returning size_t, is the
    expectation to be precise in what cannot be copied? We don't have such
    requirement for copy_*_user().

    While more intrusive, I'd rather change copy_page_from_iter_atomic()
    etc. to take a pointer where to write back an error code. If it's
    -EFAULT, retry the loop. If it's -EACCES/EPERM just bail out. Or maybe
    simply a bool set if there was an mm fault to be retried. Yet another
    option to return an -EAGAIN if it could not process the mm fault due to
    page faults being disabled.

    Happy to give this a try, unless there's a strong preference for the
    fault_in() fix-up (well, I can do both options and post them).

    --
    Catalin

    \
     
     \ /
      Last update: 2021-10-26 20:24    [W:3.048 / U:0.280 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site