| Message ID | 1453398364-22537-4-git-send-email-ross.zwisler@linux.intel.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Thu 21-01-16 10:46:02, Ross Zwisler wrote: > Several of the subtleties and assumptions of the DAX fsync/msync > implementation are not immediately obvious, so document them with comments. > > Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com> > Reported-by: Jan Kara <jack@suse.cz> Thanks, the comments really help! Just two nits below, otherwise feel free to add: Reviewed-by: Jan Kara <jack@suse.cz> > --- > fs/dax.c | 30 ++++++++++++++++++++++++++++++ > 1 file changed, 30 insertions(+) > > diff --git a/fs/dax.c b/fs/dax.c > index d589113..55ae394 100644 > --- a/fs/dax.c > +++ b/fs/dax.c > @@ -350,6 +350,13 @@ static int dax_radix_entry(struct address_space *mapping, pgoff_t index, > > if (!pmd_entry || type == RADIX_DAX_PMD) > goto dirty; > + > + /* > + * We only insert dirty PMD entries into the radix tree. This > + * means we don't need to worry about removing a dirty PTE > + * entry and inserting a clean PMD entry, thus reducing the > + * range we would flush with a follow-up fsync/msync call. > + */ May be acompany this with: WARN_ON(pmd_entry && !dirty); somewhere in dax_radix_entry()? > radix_tree_delete(&mapping->page_tree, index); > mapping->nrexceptional--; > } > @@ -912,6 +919,21 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address, > } > dax_unmap_atomic(bdev, &dax); > > + /* > + * For PTE faults we insert a radix tree entry for reads, and > + * leave it clean. Then on the first write we dirty the radix > + * tree entry via the dax_pnf_mkwrite() path. This sequence ^^^ pfn Honza
On Fri, Jan 22, 2016 at 04:01:29PM +0100, Jan Kara wrote: > On Thu 21-01-16 10:46:02, Ross Zwisler wrote: > > Several of the subtleties and assumptions of the DAX fsync/msync > > implementation are not immediately obvious, so document them with comments. > > > > Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com> > > Reported-by: Jan Kara <jack@suse.cz> > > Thanks, the comments really help! Just two nits below, otherwise feel free > to add: > > Reviewed-by: Jan Kara <jack@suse.cz> > > > --- > > fs/dax.c | 30 ++++++++++++++++++++++++++++++ > > 1 file changed, 30 insertions(+) > > > > diff --git a/fs/dax.c b/fs/dax.c > > index d589113..55ae394 100644 > > --- a/fs/dax.c > > +++ b/fs/dax.c > > @@ -350,6 +350,13 @@ static int dax_radix_entry(struct address_space *mapping, pgoff_t index, > > > > if (!pmd_entry || type == RADIX_DAX_PMD) > > goto dirty; > > + > > + /* > > + * We only insert dirty PMD entries into the radix tree. This > > + * means we don't need to worry about removing a dirty PTE > > + * entry and inserting a clean PMD entry, thus reducing the > > + * range we would flush with a follow-up fsync/msync call. > > + */ > > May be acompany this with: > > WARN_ON(pmd_entry && !dirty); > > somewhere in dax_radix_entry()? Sure, I'll add one. > > radix_tree_delete(&mapping->page_tree, index); > > mapping->nrexceptional--; > > } > > @@ -912,6 +919,21 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address, > > } > > dax_unmap_atomic(bdev, &dax); > > > > + /* > > + * For PTE faults we insert a radix tree entry for reads, and > > + * leave it clean. Then on the first write we dirty the radix > > + * tree entry via the dax_pnf_mkwrite() path. This sequence > ^^^ pfn Thanks, will fix.
--- Robert Elliott, HPE Persistent Memory > -----Original Message----- > From: Linux-nvdimm [mailto:linux-nvdimm-bounces@lists.01.org] On Behalf Of > Ross Zwisler > Sent: Friday, January 22, 2016 9:58 AM > To: Jan Kara <jack@suse.cz> > Cc: Andrew Morton <akpm@linux-foundation.org>; linux-nvdimm@lists.01.org; > Dave Chinner <david@fromorbit.com>; linux-kernel@vger.kernel.org; > Alexander Viro <viro@zeniv.linux.org.uk>; Jan Kara <jack@suse.com>; linux- > fsdevel@vger.kernel.org > Subject: Re: [PATCH v2 3/5] dax: improve documentation for fsync/msync > > On Fri, Jan 22, 2016 at 04:01:29PM +0100, Jan Kara wrote: > > On Thu 21-01-16 10:46:02, Ross Zwisler wrote: ... > > > diff --git a/fs/dax.c b/fs/dax.c > > > index d589113..55ae394 100644 > > > --- a/fs/dax.c > > > +++ b/fs/dax.c > > > @@ -350,6 +350,13 @@ static int dax_radix_entry(struct address_space > *mapping, pgoff_t index, > > > > > > if (!pmd_entry || type == RADIX_DAX_PMD) > > > goto dirty; > > > + > > > + /* > > > + * We only insert dirty PMD entries into the radix tree. This > > > + * means we don't need to worry about removing a dirty PTE > > > + * entry and inserting a clean PMD entry, thus reducing the > > > + * range we would flush with a follow-up fsync/msync call. > > > + */ > > > > May be acompany this with: > > > > WARN_ON(pmd_entry && !dirty); > > > > somewhere in dax_radix_entry()? > > Sure, I'll add one. If this is something that could trigger due to I/O traffic, please use WARN_ONCE rather than WARN_ON to avoid the risk of swamping the serial output.
diff --git a/fs/dax.c b/fs/dax.c index d589113..55ae394 100644 --- a/fs/dax.c +++ b/fs/dax.c @@ -350,6 +350,13 @@ static int dax_radix_entry(struct address_space *mapping, pgoff_t index, if (!pmd_entry || type == RADIX_DAX_PMD) goto dirty; + + /* + * We only insert dirty PMD entries into the radix tree. This + * means we don't need to worry about removing a dirty PTE + * entry and inserting a clean PMD entry, thus reducing the + * range we would flush with a follow-up fsync/msync call. + */ radix_tree_delete(&mapping->page_tree, index); mapping->nrexceptional--; } @@ -912,6 +919,21 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address, } dax_unmap_atomic(bdev, &dax); + /* + * For PTE faults we insert a radix tree entry for reads, and + * leave it clean. Then on the first write we dirty the radix + * tree entry via the dax_pnf_mkwrite() path. This sequence + * allows the dax_pfn_mkwrite() call to be simpler and avoid a + * call into get_block() to translate the pgoff to a sector in + * order to be able to create a new radix tree entry. + * + * The PMD path doesn't have an equivalent to + * dax_pfn_mkwrite(), though, so for a read followed by a + * write we traverse all the way through __dax_pmd_fault() + * twice. This means we can just skip inserting a radix tree + * entry completely on the initial read and just wait until + * the write to insert a dirty entry. + */ if (write) { error = dax_radix_entry(mapping, pgoff, dax.sector, true, true); @@ -985,6 +1007,14 @@ int dax_pfn_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf) { struct file *file = vma->vm_file; + /* + * We pass NO_SECTOR to dax_radix_entry() because we expect that a + * RADIX_DAX_PTE entry already exists in the radix tree from a + * previous call to __dax_fault(). We just want to look up that PTE + * entry using vmf->pgoff and make sure the dirty tag is set. This + * saves us from having to make a call to get_block() here to look + * up the sector. + */ dax_radix_entry(file->f_mapping, vmf->pgoff, NO_SECTOR, false, true); return VM_FAULT_NOPAGE; }
Several of the subtleties and assumptions of the DAX fsync/msync implementation are not immediately obvious, so document them with comments. Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com> Reported-by: Jan Kara <jack@suse.cz> --- fs/dax.c | 30 ++++++++++++++++++++++++++++++ 1 file changed, 30 insertions(+)