Date: Fri, 1 Oct 2021 06:07:47 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Alan Somers <asomers@freebsd.org> Cc: FreeBSD Hackers <freebsd-hackers@freebsd.org> Subject: Re: livelock in vfs_bio_getpages with vn_io_fault_uiomove Message-ID: <YVZ7g1Ub1hii5teS@kib.kiev.ua> In-Reply-To: <CAOtMX2jc_exzAPEDxV8oqdhmx0Rk25xnKk0ouaCJc4j9vo5KZQ@mail.gmail.com> References: <CAOtMX2jc_exzAPEDxV8oqdhmx0Rk25xnKk0ouaCJc4j9vo5KZQ@mail.gmail.com>
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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.
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