On 6/16/17 6:15 AM, Andrea Arcangeli wrote:
Hello Prakash,
Thanks for you response. Comments inline.
On Tue, May 09, 2017 at 01:59:34PM -0700, Prakash Sangappa wrote:
On 5/9/17 1:58 AM, Christoph Hellwig wrote:
On Mon, May 08, 2017 at 03:12:42PM -0700, prakash.sangappa wrote:
Regarding #3 as a general feature, do we want to
consider this and the complexity associated with the
implementation?
We have to. Given that no one has exclusive access to hugetlbfs
a mount option is fundamentally the wrong interface.
A hugetlbfs filesystem may need to be mounted for exclusive use by
an application. Note, recently the 'min_size' mount option was added
to hugetlbfs, which would reserve minimum number of huge pages
for that filesystem for use by an application. If the filesystem with
min size specified, is not setup for exclusive use by an application,
then the purpose of reserving huge pages is defeated. The
min_size option was for use by applications like the database.
Also, I am investigating enabling hugetlbfs mounts within user
namespace's mount namespace. That would allow an application
to mount a hugetlbfs filesystem inside a namespace exclusively for
its use, running as a non root user. For this it seems like the 'min_size'
should be subject to some user limits. Anyways, mounting inside
user namespaces is a different discussion.
So, if a filesystem has to be setup for exclusive use by an application,
then different mount options can be used for that filesystem.
Before userfaultfd I used a madvise that triggered SIGBUS. Aside from
performance that is much lower than userfaultfd because of the return
to userland, SIGBUS handling and new enter kernel to communicate
through a pipe with a memory manager, it couldn't work reliably
because you're not going to get exact information on the virtual
address that triggered the fault if the SIGBUS triggers in some random
in a copy-user of some random syscall, depending on the syscall some
random error will be returned. So it couldn't work transparently to
the app as far as syscalls and get_user_pages drivers were concerned.
Sure, seems like that would be the case if an application wants to take
some action as a result of the fault.
With your solution if you pass a corrupted pointer to a random read()
syscall you're going to get a error, but supposedly you already handle
any syscall error and stop the app.
Yes, the expectation is that the application will handle the error and
stop. This would be similar to an application passing an invalid
address to a system call.
So, in the use case for this feature, accessing the mapped address over
a hole in hugetlbfs file is invalid. The application will keep track of
the valid regions.
This is a special case because you don't care about performance and
you don't care about not returning random EFAULT errors from syscalls
like read().
Exactly.
This mount option seems non intrusive enough and hugetlbfs is quite
special already, so I'm not particularly concerned by the fact it's
one more special tweak.
If it would be enough to convert the SIGBUS into a (killable) process
hang, you could still use uffd and there would be no need to send the
uffd to a manager. You'd find the corrupting buggy process stuck in
handle_userfault().
This could be a useful feature in debug mode. However, In the normal
mode the application should exit/die.
As an alternative to the mount option we could consider adding
UFFD_FEATURE_SIGBUS that tells the handle_userfault() to simply return
VM_FAULT_SIGBUS in presence of a pagefault event. You'd still get
weird EFAULT or erratic retvals from syscalls so it would only be
usable in for your robustness feature. Then you could use UFFDIO_COPY
too to fill the memory atomically which runs faster than a page fault
(fallocate punch hole still required to zap it).
Adding a single if (ctx->feature & UFFD_FEATURE_SIGBUS) goto out,
branch for this corner case to handle_userfault() isn't great and the
hugetlbfs mount option is absolutely zero cost to the handle_userfault
which is primarily why I'm not against it.. although it's not going to
be measurable so it would be ok also to add such feature.
Sure, UFFD_FEATURE_SIGBUS would address the use case for the database
using hugetlbfs. This could be a generic API and so could be useful in
other cases as well maybe?
However for this, the userfaultfd(2) has to be opened to register. This
fd has to remain opened. Is this ok? Also, even though a monitor thread
will not be required for this particular feature, hopefully it will not
hinder future enhancements to userfaultfd.
Expectation is that the overhead of registering UFFD_FEATURE_SIGBUS is
minimal, and the registration will be done by the application ones after
every mmap() call as required, hopefully this is not required to be done
frequently. In the database use case, the registration will mainly be
done once in the beginning when mapping hugetlbfs files, so should be ok.
The mount option proposed, would give one consistent behavior for the
filesystem and will not require the application to take any additional
steps.
If implementing UFFD_FEATURE_SIGBUS is preferred instead of the mount
option, I could look into that.
Thanks,
-Prakash.
Thanks,
Andrea
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