From: Andrew Morton <akpm@linux-foundation.org> To: "Yan, Zheng" <zyan@redhat.com> Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, ceph-devel@vger.kernel.org, linux-ext4@vger.kernel.org, viro@zeniv.linux.org.uk, jlayton@redhat.com, linux-mm@kvack.org Subject: Re: [PATCH] mm: save current->journal_info before calling fault/page_mkwrite Date: Wed, 13 Dec 2017 16:59:23 -0800 [thread overview] Message-ID: <20171213165923.0ea4eb3e996b7d8bf1fff72f@linux-foundation.org> (raw) In-Reply-To: <20171213035836.916-1-zyan@redhat.com> On Wed, 13 Dec 2017 11:58:36 +0800 "Yan, Zheng" <zyan@redhat.com> wrote: > We recently got an Oops report: > > BUG: unable to handle kernel NULL pointer dereference at (null) > IP: jbd2__journal_start+0x38/0x1a2 > [...] > Call Trace: > ext4_page_mkwrite+0x307/0x52b > _ext4_get_block+0xd8/0xd8 > do_page_mkwrite+0x6e/0xd8 > handle_mm_fault+0x686/0xf9b > mntput_no_expire+0x1f/0x21e > __do_page_fault+0x21d/0x465 > dput+0x4a/0x2f7 > page_fault+0x22/0x30 > copy_user_generic_string+0x2c/0x40 > copy_page_to_iter+0x8c/0x2b8 > generic_file_read_iter+0x26e/0x845 > timerqueue_del+0x31/0x90 > ceph_read_iter+0x697/0xa33 [ceph] > hrtimer_cancel+0x23/0x41 > futex_wait+0x1c8/0x24d > get_futex_key+0x32c/0x39a > __vfs_read+0xe0/0x130 > vfs_read.part.1+0x6c/0x123 > handle_mm_fault+0x831/0xf9b > __fget+0x7e/0xbf > SyS_read+0x4d/0xb5 > > The reason is that page fault can happen when one filesystem copies > data from/to userspace, the filesystem may set current->journal_info. > If the userspace memory is mapped to a file on another filesystem, > the later filesystem may also want to use current->journal_info. > whoops. A cc:stable will be needed here... A filesystem doesn't "copy data from/to userspace". I assume here we're referring to a read() where the source is a pagecache page for filesystem A and the destination is a MAP_SHARED page in filesystem B? But in that case I don't see why filesystem A would have a live ->journal_info? It's just doing a read. So can we please have more detailed info on the exact scenario here? > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -2347,12 +2347,22 @@ static int do_page_mkwrite(struct vm_fault *vmf) > { > int ret; > struct page *page = vmf->page; > + void *old_journal_info = current->journal_info; > unsigned int old_flags = vmf->flags; > > + /* > + * If the fault happens during read_iter() copies data to > + * userspace, filesystem may have set current->journal_info. > + * If the userspace memory is mapped to a file on another > + * filesystem, page_mkwrite() of the later filesystem may > + * want to access/modify current->journal_info. > + */ > + current->journal_info = NULL; > vmf->flags = FAULT_FLAG_WRITE|FAULT_FLAG_MKWRITE; > > ret = vmf->vma->vm_ops->page_mkwrite(vmf); > - /* Restore original flags so that caller is not surprised */ > + /* Restore original journal_info and flags */ > + current->journal_info = old_journal_info; > vmf->flags = old_flags; > if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE))) > return ret; > @@ -3191,9 +3201,20 @@ static int do_anonymous_page(struct vm_fault *vmf) > static int __do_fault(struct vm_fault *vmf) > { > struct vm_area_struct *vma = vmf->vma; > + void *old_journal_info = current->journal_info; > int ret; > > + /* > + * If the fault happens during write_iter() copies data from > + * userspace, filesystem may have set current->journal_info. > + * If the userspace memory is mapped to a file on another > + * filesystem, fault handler of the later filesystem may want > + * to access/modify current->journal_info. > + */ > + current->journal_info = NULL; > ret = vma->vm_ops->fault(vmf); > + /* Restore original journal_info */ > + current->journal_info = old_journal_info; > if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY | > VM_FAULT_DONE_COW))) > return ret; Can you explain why you chose these two sites? Rather than, for example, way up in handle_mm_fault()? It's hard to believe that a fault handler will alter ->journal_info if it is handling a read fault, so perhaps we only need to do this for a write fault? Although such an optimization probably isn't worthwhile. The whole thing is only about three instructions.
WARNING: multiple messages have this Message-ID (diff)
From: Andrew Morton <akpm@linux-foundation.org> To: "Yan, Zheng" <zyan@redhat.com> Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, ceph-devel@vger.kernel.org, linux-ext4@vger.kernel.org, viro@zeniv.linux.org.uk, jlayton@redhat.com, linux-mm@kvack.org Subject: Re: [PATCH] mm: save current->journal_info before calling fault/page_mkwrite Date: Wed, 13 Dec 2017 16:59:23 -0800 [thread overview] Message-ID: <20171213165923.0ea4eb3e996b7d8bf1fff72f@linux-foundation.org> (raw) In-Reply-To: <20171213035836.916-1-zyan@redhat.com> On Wed, 13 Dec 2017 11:58:36 +0800 "Yan, Zheng" <zyan@redhat.com> wrote: > We recently got an Oops report: > > BUG: unable to handle kernel NULL pointer dereference at (null) > IP: jbd2__journal_start+0x38/0x1a2 > [...] > Call Trace: > ext4_page_mkwrite+0x307/0x52b > _ext4_get_block+0xd8/0xd8 > do_page_mkwrite+0x6e/0xd8 > handle_mm_fault+0x686/0xf9b > mntput_no_expire+0x1f/0x21e > __do_page_fault+0x21d/0x465 > dput+0x4a/0x2f7 > page_fault+0x22/0x30 > copy_user_generic_string+0x2c/0x40 > copy_page_to_iter+0x8c/0x2b8 > generic_file_read_iter+0x26e/0x845 > timerqueue_del+0x31/0x90 > ceph_read_iter+0x697/0xa33 [ceph] > hrtimer_cancel+0x23/0x41 > futex_wait+0x1c8/0x24d > get_futex_key+0x32c/0x39a > __vfs_read+0xe0/0x130 > vfs_read.part.1+0x6c/0x123 > handle_mm_fault+0x831/0xf9b > __fget+0x7e/0xbf > SyS_read+0x4d/0xb5 > > The reason is that page fault can happen when one filesystem copies > data from/to userspace, the filesystem may set current->journal_info. > If the userspace memory is mapped to a file on another filesystem, > the later filesystem may also want to use current->journal_info. > whoops. A cc:stable will be needed here... A filesystem doesn't "copy data from/to userspace". I assume here we're referring to a read() where the source is a pagecache page for filesystem A and the destination is a MAP_SHARED page in filesystem B? But in that case I don't see why filesystem A would have a live ->journal_info? It's just doing a read. So can we please have more detailed info on the exact scenario here? > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -2347,12 +2347,22 @@ static int do_page_mkwrite(struct vm_fault *vmf) > { > int ret; > struct page *page = vmf->page; > + void *old_journal_info = current->journal_info; > unsigned int old_flags = vmf->flags; > > + /* > + * If the fault happens during read_iter() copies data to > + * userspace, filesystem may have set current->journal_info. > + * If the userspace memory is mapped to a file on another > + * filesystem, page_mkwrite() of the later filesystem may > + * want to access/modify current->journal_info. > + */ > + current->journal_info = NULL; > vmf->flags = FAULT_FLAG_WRITE|FAULT_FLAG_MKWRITE; > > ret = vmf->vma->vm_ops->page_mkwrite(vmf); > - /* Restore original flags so that caller is not surprised */ > + /* Restore original journal_info and flags */ > + current->journal_info = old_journal_info; > vmf->flags = old_flags; > if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE))) > return ret; > @@ -3191,9 +3201,20 @@ static int do_anonymous_page(struct vm_fault *vmf) > static int __do_fault(struct vm_fault *vmf) > { > struct vm_area_struct *vma = vmf->vma; > + void *old_journal_info = current->journal_info; > int ret; > > + /* > + * If the fault happens during write_iter() copies data from > + * userspace, filesystem may have set current->journal_info. > + * If the userspace memory is mapped to a file on another > + * filesystem, fault handler of the later filesystem may want > + * to access/modify current->journal_info. > + */ > + current->journal_info = NULL; > ret = vma->vm_ops->fault(vmf); > + /* Restore original journal_info */ > + current->journal_info = old_journal_info; > if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY | > VM_FAULT_DONE_COW))) > return ret; Can you explain why you chose these two sites? Rather than, for example, way up in handle_mm_fault()? It's hard to believe that a fault handler will alter ->journal_info if it is handling a read fault, so perhaps we only need to do this for a write fault? Although such an optimization probably isn't worthwhile. The whole thing is only about three instructions. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
next prev parent reply other threads:[~2017-12-14 0:59 UTC|newest] Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top 2017-12-13 3:58 [PATCH] mm: save current->journal_info before calling fault/page_mkwrite Yan, Zheng 2017-12-13 13:53 ` Amon Ott 2017-12-14 0:59 ` Andrew Morton [this message] 2017-12-14 0:59 ` Andrew Morton 2017-12-14 2:09 ` Yan, Zheng 2017-12-14 2:09 ` Yan, Zheng 2017-12-14 2:18 ` Andrew Morton 2017-12-14 2:18 ` Andrew Morton 2017-12-14 2:20 ` Yan, Zheng 2017-12-14 2:20 ` Yan, Zheng 2017-12-14 2:30 ` Andrew Morton 2017-12-14 2:30 ` Andrew Morton
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