git: 981e9c148600 - stable/13 - vfs: [2/2] fix stalls in vnode reclaim by only counting attempts
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Date: Tue, 15 Mar 2022 21:15:35 UTC
The branch stable/13 has been updated by mjg: URL: https://cgit.FreeBSD.org/src/commit/?id=981e9c148600a55c7db168a29fa11a8070305cc4 commit 981e9c148600a55c7db168a29fa11a8070305cc4 Author: Mateusz Guzik <mjg@FreeBSD.org> AuthorDate: 2022-03-07 10:38:17 +0000 Commit: Mateusz Guzik <mjg@FreeBSD.org> CommitDate: 2022-03-15 21:12:50 +0000 vfs: [2/2] fix stalls in vnode reclaim by only counting attempts ... and ignoring if they succeded, which matches historical behavior. Reported by: pho (cherry picked from commit 3a4c5dab9266fac93a5cb22c7cee3938466aedea) --- sys/kern/vfs_subr.c | 25 +++++++++++++++++++++++-- 1 file changed, 23 insertions(+), 2 deletions(-) diff --git a/sys/kern/vfs_subr.c b/sys/kern/vfs_subr.c index 5c5faf455f2b..d27e7a15d886 100644 --- a/sys/kern/vfs_subr.c +++ b/sys/kern/vfs_subr.c @@ -1307,8 +1307,29 @@ vnlru_free_impl(int count, struct vfsops *mnt_op, struct vnode *mvp) TAILQ_REMOVE(&vnode_list, mvp, v_vnodelist); TAILQ_INSERT_AFTER(&vnode_list, vp, mvp, v_vnodelist); mtx_unlock(&vnode_list_mtx); - if (vtryrecycle(vp) == 0) - count--; + /* + * FIXME: ignores the return value, meaning it may be nothing + * got recycled but it claims otherwise to the caller. + * + * Originally the value started being ignored in 2005 with + * 114a1006a8204aa156e1f9ad6476cdff89cada7f . + * + * Respecting the value can run into significant stalls if most + * vnodes belong to one file system and it has writes + * suspended. In presence of many threads and millions of + * vnodes they keep contending on the vnode_list_mtx lock only + * to find vnodes they can't recycle. + * + * The solution would be to pre-check if the vnode is likely to + * be recycle-able, but it needs to happen with the + * vnode_list_mtx lock held. This runs into a problem where + * VOP_GETWRITEMOUNT (currently needed to find out about if + * writes are frozen) can take locks which LOR against it. + * + * Check nullfs for one example (null_getwritemount). + */ + vtryrecycle(vp); + count--; mtx_lock(&vnode_list_mtx); vp = mvp; }