From: rdodgen@gmail.com To: tytso@mit.edu, linux-ext4@vger.kernel.org, ross.zwisler@linux.intel.com Cc: Randy Dodgen <dodgen@google.com>, linux-nvdimm@lists.01.org Subject: [PATCH v2] Fix ext4 fault handling when mounted with -o dax,ro Date: Tue, 22 Aug 2017 20:37:04 -0700 [thread overview] Message-ID: <bea6897723e5bf09d893b2ca40263a894a284a5a.1503459187.git.dodgen@google.com> (raw) In-Reply-To: <20170822222434.146233-1-dodgen@google.com> From: Randy Dodgen <dodgen@google.com> If an ext4 filesystem is mounted with both the DAX and read-only options, executables on that filesystem will fail to start (claiming 'Segmentation fault') due to the fault handler returning VM_FAULT_SIGBUS. This is due to the DAX fault handler (see ext4_dax_huge_fault) attempting to write to the journal when FAULT_FLAG_WRITE is set. This is the wrong behavior for write faults which will lead to a COW page; in particular, this fails for readonly mounts. This changes replicates some check from dax_iomap_fault to more precisely reason about when a journal-write is needed. It might be the case that this could be better handled in ext4_iomap_begin / ext4_iomap_end (called via iomap_ops inside dax_iomap_fault). These is some overlap already (e.g. grabbing journal handles). Signed-off-by: Randy Dodgen <dodgen@google.com> --- I'm resending for some DMARC-proofing (thanks Ted for the explanation), a missing Signed-off-by, and some extra cc's. Oops! fs/ext4/file.c | 26 +++++++++++++++++++++++++- 1 file changed, 25 insertions(+), 1 deletion(-) diff --git a/fs/ext4/file.c b/fs/ext4/file.c index 0d7cf0cc9b87..d512fb85a3e3 100644 --- a/fs/ext4/file.c +++ b/fs/ext4/file.c @@ -279,7 +279,31 @@ static int ext4_dax_huge_fault(struct vm_fault *vmf, handle_t *handle = NULL; struct inode *inode = file_inode(vmf->vma->vm_file); struct super_block *sb = inode->i_sb; - bool write = vmf->flags & FAULT_FLAG_WRITE; + bool write; + + /* + * We have to distinguish real writes from writes which will result in a + * COW page + * - COW writes need to fall-back to installing PTEs. See + * dax_iomap_pmd_fault. + * - COW writes should *not* poke the journal (the file will not be + * changed). Doing so would cause unintended failures when mounted + * read-only. + */ + if (pe_size == PE_SIZE_PTE) { + /* See dax_iomap_pte_fault. */ + write = (vmf->flags & FAULT_FLAG_WRITE) && !vmf->cow_page; + } else if (pe_size == PE_SIZE_PMD) { + /* See dax_iomap_pmd_fault. */ + write = vmf->flags & FAULT_FLAG_WRITE; + if (write && !(vmf->vma->vm_flags & VM_SHARED)) { + split_huge_pmd(vmf->vma, vmf->pmd, vmf->address); + count_vm_event(THP_FAULT_FALLBACK); + return VM_FAULT_FALLBACK; + } + } else { + return VM_FAULT_FALLBACK; + } if (write) { sb_start_pagefault(sb); -- 2.14.1.480.gb18f417b89-goog _______________________________________________ Linux-nvdimm mailing list Linux-nvdimm@lists.01.org https://lists.01.org/mailman/listinfo/linux-nvdimm
WARNING: multiple messages have this Message-ID (diff)
From: rdodgen@gmail.com To: tytso@mit.edu, linux-ext4@vger.kernel.org, ross.zwisler@linux.intel.com Cc: Randy Dodgen <dodgen@google.com>, linux-nvdimm@lists.01.org Subject: [PATCH v2] Fix ext4 fault handling when mounted with -o dax,ro Date: Tue, 22 Aug 2017 20:37:04 -0700 [thread overview] Message-ID: <bea6897723e5bf09d893b2ca40263a894a284a5a.1503459187.git.dodgen@google.com> (raw) In-Reply-To: <20170822222434.146233-1-dodgen@google.com> From: Randy Dodgen <dodgen@google.com> If an ext4 filesystem is mounted with both the DAX and read-only options, executables on that filesystem will fail to start (claiming 'Segmentation fault') due to the fault handler returning VM_FAULT_SIGBUS. This is due to the DAX fault handler (see ext4_dax_huge_fault) attempting to write to the journal when FAULT_FLAG_WRITE is set. This is the wrong behavior for write faults which will lead to a COW page; in particular, this fails for readonly mounts. This changes replicates some check from dax_iomap_fault to more precisely reason about when a journal-write is needed. It might be the case that this could be better handled in ext4_iomap_begin / ext4_iomap_end (called via iomap_ops inside dax_iomap_fault). These is some overlap already (e.g. grabbing journal handles). Signed-off-by: Randy Dodgen <dodgen@google.com> --- I'm resending for some DMARC-proofing (thanks Ted for the explanation), a missing Signed-off-by, and some extra cc's. Oops! fs/ext4/file.c | 26 +++++++++++++++++++++++++- 1 file changed, 25 insertions(+), 1 deletion(-) diff --git a/fs/ext4/file.c b/fs/ext4/file.c index 0d7cf0cc9b87..d512fb85a3e3 100644 --- a/fs/ext4/file.c +++ b/fs/ext4/file.c @@ -279,7 +279,31 @@ static int ext4_dax_huge_fault(struct vm_fault *vmf, handle_t *handle = NULL; struct inode *inode = file_inode(vmf->vma->vm_file); struct super_block *sb = inode->i_sb; - bool write = vmf->flags & FAULT_FLAG_WRITE; + bool write; + + /* + * We have to distinguish real writes from writes which will result in a + * COW page + * - COW writes need to fall-back to installing PTEs. See + * dax_iomap_pmd_fault. + * - COW writes should *not* poke the journal (the file will not be + * changed). Doing so would cause unintended failures when mounted + * read-only. + */ + if (pe_size == PE_SIZE_PTE) { + /* See dax_iomap_pte_fault. */ + write = (vmf->flags & FAULT_FLAG_WRITE) && !vmf->cow_page; + } else if (pe_size == PE_SIZE_PMD) { + /* See dax_iomap_pmd_fault. */ + write = vmf->flags & FAULT_FLAG_WRITE; + if (write && !(vmf->vma->vm_flags & VM_SHARED)) { + split_huge_pmd(vmf->vma, vmf->pmd, vmf->address); + count_vm_event(THP_FAULT_FALLBACK); + return VM_FAULT_FALLBACK; + } + } else { + return VM_FAULT_FALLBACK; + } if (write) { sb_start_pagefault(sb); -- 2.14.1.480.gb18f417b89-goog
next prev parent reply other threads:[~2017-08-23 3:35 UTC|newest] Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top 2017-08-22 22:24 [PATCH] Fix ext4 fault handling when mounted with -o dax,ro Randy Dodgen 2017-08-23 3:37 ` rdodgen [this message] 2017-08-23 3:37 ` [PATCH v2] " rdodgen 2017-08-23 16:38 ` Ross Zwisler 2017-08-23 16:38 ` Ross Zwisler 2017-08-23 20:11 ` Randy Dodgen 2017-08-23 20:11 ` Randy Dodgen 2017-08-23 21:26 ` [PATCH v3] " rdodgen 2017-08-23 21:26 ` rdodgen 2017-08-24 14:45 ` Jan Kara 2017-08-24 14:45 ` Jan Kara 2017-08-24 15:11 ` Ross Zwisler 2017-08-24 15:11 ` Ross Zwisler 2017-08-24 16:01 ` Christoph Hellwig 2017-08-24 16:01 ` Christoph Hellwig 2017-08-24 20:57 ` Theodore Ts'o 2017-08-24 20:57 ` Theodore Ts'o 2017-08-25 7:28 ` Christoph Hellwig 2017-08-25 7:28 ` Christoph Hellwig 2017-08-29 21:20 ` Christoph Hellwig 2017-08-29 21:20 ` Christoph Hellwig 2017-08-29 21:37 ` Ross Zwisler 2017-08-29 21:37 ` Ross Zwisler 2017-08-29 22:07 ` Randy Dodgen 2017-08-29 22:07 ` Randy Dodgen 2017-08-29 22:37 ` [fstests PATCH] generic: add test for executables on read-only DAX mounts Ross Zwisler 2017-08-29 22:37 ` Ross Zwisler 2017-08-30 10:59 ` Eryu Guan 2017-08-30 10:59 ` Eryu Guan 2017-08-31 4:01 ` Ross Zwisler 2017-08-31 4:01 ` Ross Zwisler 2017-08-31 4:01 ` Ross Zwisler 2017-08-31 4:09 ` [fstests v2] " Ross Zwisler 2017-08-31 4:09 ` Ross Zwisler 2017-08-30 14:51 ` [fstests PATCH] " Christoph Hellwig 2017-08-30 14:51 ` Christoph Hellwig 2017-08-30 14:51 ` Christoph Hellwig 2017-08-31 4:02 ` Ross Zwisler 2017-08-31 4:02 ` Ross Zwisler 2017-08-31 4:02 ` Ross Zwisler 2017-08-31 13:09 ` Christoph Hellwig 2017-08-31 13:09 ` Christoph Hellwig 2017-08-24 19:26 ` [PATCH v3] Fix ext4 fault handling when mounted with -o dax,ro Theodore Ts'o 2017-08-24 19:26 ` Theodore Ts'o
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