Re: livelock in vfs_bio_getpages with vn_io_fault_uiomove
Date: Fri, 01 Oct 2021 03:53:18 UTC
On Thu, Sep 30, 2021 at 09:33:24PM -0600, Alan Somers wrote: > On Thu, Sep 30, 2021 at 9:08 PM Konstantin Belousov <kostikbel@gmail.com> wrote: > > > > On Thu, Sep 30, 2021 at 03:11:02PM -0600, Alan Somers wrote: > > > I'm trying to adapt fusefs to use vn_io_fault_uiomove to fix the > > > deadlock described in the comments above vn_io_fault_doio [^1]. I can > > > reproduce the deadlock readily enough, and the fix seems simple enough > > > in other filesystems [^2][^3]. But when I try to apply the same fix > > > to fusefs, the deadlock changes into a livelock. vfs_bio_getpages > > > loops infinitely because it reaches the "redo = true" state. But on > > > looping, it never attempts to read from fusefs again. Instead, > > > breadn_flags returns 0 without ever calling bufstrategy, from which I > > > infer that getblkx returned a block with B_CACHE set. Despite that, > > > at least one of the requested pages in vfs_bio_getpages fails the > > > vm_page_all_valid(ma[i]) check. Debugging further is wandering > > > outside my areas of expertise. Could somebody please give me a tip? > > > What is supposed to mark those pages as valid? Are there any other > > > undocumented conditions needed to use vn_io_fault_uiomove that msdosfs > > > and nfscl just happened to already meet? > > > > > > Grateful for any help, > > > -Alan > > > > > > [^1] https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=238340 > > > [^2] https://github.com/freebsd/freebsd-src/commit/2aa3944510b50cbe6999344985a5a9c3208063b2 > > > [^3] https://github.com/freebsd/freebsd-src/commit/ddfc47fdc98460b757c6d1dbe4562a0a339f228b > > > > Perhaps first you need to confirm that vfs_bio_getpages() sees a buffer > > with B_CACHE set but some pages still not fully valid (except the last > > page that is allowed to have invalid tail at EOF). > > > > When buffer strategy read returns, vfs_vmio_iodone() updates the pages > > validity bitmap according to the b_resid field of the buffer. Look there, > > might be you did not set it properly. Practically, both b_resid and b_bcount > > must be correctly set after io. > > > > In fact, after writing out all that, I realized that I am confused > > by your question. vn_io_fault_uiomove() needs to be used from > > VOP_READ/VOP_WRITE. If filesystem utilizes buffer cache, then there is > > a VOP_STRATEGY() implementation that fullfils the buffer cache requests > > for buffers reads and writes. VOP_READ and VOP_STRATEGY simply occurs > > at very different layers of the io stack. Typically, VOP_READ() does > > bread() which might trigger VOP_STRATEGY() to get the buffer, and then > > it performs vn_io_fault() to move data from locked buffer to userspace. > > > > The fact that your addition of vn_io_fault breaks something in VOP_STRATEGY() > > does not make sense. > > Ahh, that last piece of information is useful. In fusefs, both > VOP_READ and VOP_STRATEGY can end up going through the same path, > depending on cache settings, O_DIRECT, etc. And during a VOP_STRATEGY > read, fusefs needs to use uiomove in order to move data not into the > user's buffer, but from the fuse daemon into the kernel. But that's > not the cause of the livelock, because whether I use uiomove or > vn_io_fault_uiomove during VOP_STRATEGY I still get the same livelock. > I'll check b_bcount and b_resid tomorrow. But uiomove call from VOP_STRATEGY() to copy user data into kernel buffer is not prepared for vn_io_fault. I suspect that what happens there is the following: - top level of syscall, like read(2), does vn_read() - vn_read() checks conditions and goes through vn_io_fault, calling VOP_READ() there it prepares prefaulted held pages _for userspace buffer from read(2)_ - your VOP_READ() calls into VOP_STRATEGY() that tries to copy data into kernel. If you use vn_io_fault_uiomove() at this point, it wrongly consumes held pages for unrelated userspace buffer Even if there is some additional bug with b_resid, I suspect that the end result is the mess anyway. You should not use vn_io_fault for recursive accesses to userspace, only for VOP_READ/VOP_WRITE level accesses.