Date: Mon, 27 May 2019 14:21:55 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Alexey Dokuchaev <danfe@freebsd.org> Cc: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r232071 - head/sys/vm Message-ID: <20190527112155.GZ2748@kib.kiev.ua> In-Reply-To: <20190527095256.GA52203@FreeBSD.org> References: <201202232107.q1NL7GHi023139@svn.freebsd.org> <20190527095256.GA52203@FreeBSD.org>
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On Mon, May 27, 2019 at 09:52:56AM +0000, Alexey Dokuchaev wrote: > On Thu, Feb 23, 2012 at 09:07:16PM +0000, Konstantin Belousov wrote: > > New Revision: 232071 > > URL: http://svn.freebsd.org/changeset/base/232071 > > > > Log: > > Account the writeable shared mappings backed by file in the vnode > > v_writecount. Keep the amount of the virtual address space used by > > the mappings in the new vm_object un_pager.vnp.writemappings > > counter. The vnode v_writecount is incremented when writemappings gets > > non-zero value, and decremented when writemappings is returned to > > zero. > > > > Writeable shared vnode-backed mappings are accounted for in vm_mmap(), > > and vm_map_insert() is instructed to set MAP_ENTRY_VN_WRITECNT flag on > > the created map entry. During deferred map entry deallocation, > > vm_map_process_deferred() checks for MAP_ENTRY_VN_WRITECOUNT and > > decrements writemappings for the vm object. > > > > Now, the writeable mount cannot be demoted to read-only while > > writeable shared mappings of the vnodes from the mount point > > exist. Also, execve(2) fails for such files with ETXTBUSY, as it > > should be. > > > > ... > > Modified: head/sys/vm/vnode_pager.c > > ============================================================================== > > --- head/sys/vm/vnode_pager.c Thu Feb 23 20:58:52 2012 (r232070) > > +++ head/sys/vm/vnode_pager.c Thu Feb 23 21:07:16 2012 (r232071) > > @@ -1215,3 +1222,81 @@ vnode_pager_undirty_pages(vm_page_t *ma, > > } > > VM_OBJECT_UNLOCK(obj); > > } > > + > > +void > > +vnode_pager_update_writecount(vm_object_t object, vm_offset_t start, > > + vm_offset_t end) > > So, it is first `start, then `end', but below... > > > +{ > > + struct vnode *vp; > > + vm_ooffset_t old_wm; > > + > > + ... > > +} > > + > > +void > > +vnode_pager_release_writecount(vm_object_t object, vm_offset_t start, > > + vm_offset_t end) > > +{ > > + struct vnode *vp; > > + struct mount *mp; > > + vm_offset_t inc; > > + int vfslocked; > > + > > + VM_OBJECT_LOCK(object); > > + > > + /* > > + * First, recheck the object type to account for the race when > > + * the vnode is reclaimed. > > + */ > > + if (object->type != OBJT_VNODE) { > > + VM_OBJECT_UNLOCK(object); > > + return; > > + } > > + > > + /* > > + * Optimize for the case when writemappings is not going to > > + * zero. > > + */ > > + inc = end - start; > > + if (object->un_pager.vnp.writemappings != inc) { > > + object->un_pager.vnp.writemappings -= inc; > > + VM_OBJECT_UNLOCK(object); > > + return; > > + } > > + > > + vp = object->handle; > > + vhold(vp); > > + VM_OBJECT_UNLOCK(object); > > + vfslocked = VFS_LOCK_GIANT(vp->v_mount); > > + mp = NULL; > > + vn_start_write(vp, &mp, V_WAIT); > > + vn_lock(vp, LK_EXCLUSIVE | LK_RETRY); > > + > > + /* > > + * Decrement the object's writemappings, by swapping the start > > + * and end arguments for vnode_pager_update_writecount(). If > > + * there was not a race with vnode reclaimation, then the > > + * vnode's v_writecount is decremented. > > + */ > > + vnode_pager_update_writecount(object, end, start); > > ... here, first `end' is passed, then `start'. Is this intentional? Did you read the comment right before the call ? > > PVS Studio complains: > > /usr/src/sys/vm/vnode_pager.c:1584:1: warning: V764 Possible > incorrect order of arguments passed to 'vnode_pager_update_writecount' > function: 'end' and 'start'. > > > + VOP_UNLOCK(vp, 0); > > + vdrop(vp); > > + if (mp != NULL) > > + vn_finished_write(mp); > > + VFS_UNLOCK_GIANT(vfslocked); > > +}
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