Messages in this thread | | | From | (Eric W. Biederman) | Date | Wed, 09 Apr 2014 18:36:12 -0700 | Subject | Re: [RFC][PATCH] vfs: In mntput run deactivate_super on a shallow stack. |
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Al Viro <viro@ZenIV.linux.org.uk> writes:
> On Wed, Apr 09, 2014 at 03:58:25PM -0700, Eric W. Biederman wrote: >> >> mntput as part of pathput is called from all over the vfs sometimes as >> in the case of symlink chasing from some rather deep call chains. >> During filesystem unmount with the right set of races those innocuous >> little mntput calls that take very little stack space can become calls >> become mosters calling deactivate_super that can take up 3k or more of >> stack space as syncrhonous filesystem I/O is performed, through >> multiple levels of the I/O stack. >> >> Avoid deactivate_super being called from a deep stack by converting >> mntput to use task_work_add when the mnt_count goes to 0. The >> filesystem is still unmounted synchronously preserving the semantics >> that system calls like umount require. > > Careful. For one thing, you've just introduced a massive leak in knfsd > and any other kernel thread that might do mntput(). task_work_add() > makes no sense there - there is no userland to return to. For another, > in things like cleanup of failing modprobe we might end up delaying fs > shutdown too much. So it's not that simple, unfortunately.
Unfortunately.
> I agree that fs shutdown is better dealt with on mostly empty stack, of > course - moreover, done right that has a potential to make mntput() > safe in atomic contexts (there's also acct_auto_close_mnt() to deal > with; that might take some work to get right, but I think it's not > fatal).
I am slowly digging into this.
With this patch I was was able to do an A/B comparison of what the stack cost on my unmounting my minimal ext4 filesystem from d_invalidate called with in a context with maximum symlink recursion depth, without and without a changed mntput.
I used sysfs instead of nfs to mount my minimal ext4 filesystem on as I was a lazy bum and didn't have a nfs server setup handy.
With just my detach_mounts branched merged into 3.15-rc0 I saw 4880 stack bytes left before calling detach_mounts from d_invalidate I saw 3904 stack bytes left after calling detach_mounts from d_invalidate
Which means in practice unmounting my mininal ext4 filesystem image only used 976 additional bytes of stack.
With the same kernel plus my change to mntput I saw 4880 stack bytes left before calling detach_mounts from d_invalidate I saw 4880 stack bytes left after calling detach_mounts from d_invalidate
Which at least confirms that a change to mntput is enough to make deep stacks safe.
With 3904 bytes of headroom from ext4 I may have to measure some of the nastier cases just to be certain there actually is a problem here.
Eric
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