freebsd-dev/module/zfs/zpl_file.c
Brian Behlendorf 3c0e5c0f45 Cleanup mmap(2) writes
While the existing implementation of .writepage()/zpl_putpage() was
functional it was not entirely correct.  In particular, it would move
dirty pages in to a clean state simply after copying them in to the
ARC cache.  This would result in the pages being lost if the system
were to crash enough though the Linux VFS believed them to be safe on
stable storage.

Since at the moment virtually all I/O, except mmap(2), bypasses the
page cache this isn't as bad as it sounds.  However, as hopefully
start using the page cache more getting this right becomes more
important so it's good to improve this now.

This patch takes a big step in that direction by updating the code
to correctly move dirty pages through a writeback phase before they
are marked clean.  When a dirty page is copied in to the ARC it will
now be set in writeback and a completion callback is registered with
the transaction.  The page will stay in writeback until the dmu runs
the completion callback indicating the page is on stable storage.
At this point the page can be safely marked clean.

This process is normally entirely asynchronous and will be repeated
for every dirty page.  This may initially sound inefficient but most
of these pages will end up in a few txgs.  That means when they are
eventually written to disk they should be nicely batched.  However,
there is room for improvement.  It may still be desirable to batch
up the pages in to larger writes for the dmu.  This would reduce
the number of callbacks and small 4k buffer required by the ARC.

Finally, if the caller requires that the I/O be done synchronously
by setting WB_SYNC_ALL or if ZFS_SYNC_ALWAYS is set.  Then the I/O
will trigger a zil_commit() to flush the data to stable storage.
At which point the registered callbacks will be run leaving the
date safe of disk and marked clean before returning from .writepage.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
2011-08-02 10:34:55 -07:00

416 lines
10 KiB
C

/*
* CDDL HEADER START
*
* The contents of this file are subject to the terms of the
* Common Development and Distribution License (the "License").
* You may not use this file except in compliance with the License.
*
* You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
* or http://www.opensolaris.org/os/licensing.
* See the License for the specific language governing permissions
* and limitations under the License.
*
* When distributing Covered Code, include this CDDL HEADER in each
* file and include the License file at usr/src/OPENSOLARIS.LICENSE.
* If applicable, add the following below this CDDL HEADER, with the
* fields enclosed by brackets "[]" replaced with your own identifying
* information: Portions Copyright [yyyy] [name of copyright owner]
*
* CDDL HEADER END
*/
/*
* Copyright (c) 2011, Lawrence Livermore National Security, LLC.
*/
#include <sys/zfs_vfsops.h>
#include <sys/zfs_vnops.h>
#include <sys/zfs_znode.h>
#include <sys/zpl.h>
static int
zpl_open(struct inode *ip, struct file *filp)
{
cred_t *cr = CRED();
int error;
crhold(cr);
error = -zfs_open(ip, filp->f_mode, filp->f_flags, cr);
crfree(cr);
ASSERT3S(error, <=, 0);
if (error)
return (error);
return generic_file_open(ip, filp);
}
static int
zpl_release(struct inode *ip, struct file *filp)
{
cred_t *cr = CRED();
int error;
crhold(cr);
error = -zfs_close(ip, filp->f_flags, cr);
crfree(cr);
ASSERT3S(error, <=, 0);
return (error);
}
static int
zpl_readdir(struct file *filp, void *dirent, filldir_t filldir)
{
struct dentry *dentry = filp->f_path.dentry;
cred_t *cr = CRED();
int error;
crhold(cr);
error = -zfs_readdir(dentry->d_inode, dirent, filldir,
&filp->f_pos, cr);
crfree(cr);
ASSERT3S(error, <=, 0);
return (error);
}
/*
* 2.6.35 API change,
* As of 2.6.35 the dentry argument to the .fsync() vfs hook was deemed
* redundant. The dentry is still accessible via filp->f_path.dentry,
* and we are guaranteed that filp will never be NULL.
*
* 2.6.34 API change,
* Prior to 2.6.34 the nfsd kernel server would pass a NULL file struct *
* to the .fsync() hook. For this reason, we must be careful not to use
* filp unconditionally in the 3 argument case.
*/
#ifdef HAVE_2ARGS_FSYNC
static int
zpl_fsync(struct file *filp, int datasync)
{
struct dentry *dentry = filp->f_path.dentry;
#else
static int
zpl_fsync(struct file *filp, struct dentry *dentry, int datasync)
{
#endif /* HAVE_2ARGS_FSYNC */
cred_t *cr = CRED();
int error;
crhold(cr);
error = -zfs_fsync(dentry->d_inode, datasync, cr);
crfree(cr);
ASSERT3S(error, <=, 0);
return (error);
}
ssize_t
zpl_read_common(struct inode *ip, const char *buf, size_t len, loff_t pos,
uio_seg_t segment, int flags, cred_t *cr)
{
int error;
struct iovec iov;
uio_t uio;
iov.iov_base = (void *)buf;
iov.iov_len = len;
uio.uio_iov = &iov;
uio.uio_resid = len;
uio.uio_iovcnt = 1;
uio.uio_loffset = pos;
uio.uio_limit = MAXOFFSET_T;
uio.uio_segflg = segment;
error = -zfs_read(ip, &uio, flags, cr);
if (error < 0)
return (error);
return (len - uio.uio_resid);
}
static ssize_t
zpl_read(struct file *filp, char __user *buf, size_t len, loff_t *ppos)
{
cred_t *cr = CRED();
ssize_t read;
crhold(cr);
read = zpl_read_common(filp->f_mapping->host, buf, len, *ppos,
UIO_USERSPACE, filp->f_flags, cr);
crfree(cr);
if (read < 0)
return (read);
*ppos += read;
return (read);
}
ssize_t
zpl_write_common(struct inode *ip, const char *buf, size_t len, loff_t pos,
uio_seg_t segment, int flags, cred_t *cr)
{
int error;
struct iovec iov;
uio_t uio;
iov.iov_base = (void *)buf;
iov.iov_len = len;
uio.uio_iov = &iov;
uio.uio_resid = len,
uio.uio_iovcnt = 1;
uio.uio_loffset = pos;
uio.uio_limit = MAXOFFSET_T;
uio.uio_segflg = segment;
error = -zfs_write(ip, &uio, flags, cr);
if (error < 0)
return (error);
return (len - uio.uio_resid);
}
static ssize_t
zpl_write(struct file *filp, const char __user *buf, size_t len, loff_t *ppos)
{
cred_t *cr = CRED();
ssize_t wrote;
crhold(cr);
wrote = zpl_write_common(filp->f_mapping->host, buf, len, *ppos,
UIO_USERSPACE, filp->f_flags, cr);
crfree(cr);
if (wrote < 0)
return (wrote);
*ppos += wrote;
return (wrote);
}
/*
* It's worth taking a moment to describe how mmap is implemented
* for zfs because it differs considerably from other Linux filesystems.
* However, this issue is handled the same way under OpenSolaris.
*
* The issue is that by design zfs bypasses the Linux page cache and
* leaves all caching up to the ARC. This has been shown to work
* well for the common read(2)/write(2) case. However, mmap(2)
* is problem because it relies on being tightly integrated with the
* page cache. To handle this we cache mmap'ed files twice, once in
* the ARC and a second time in the page cache. The code is careful
* to keep both copies synchronized.
*
* When a file with an mmap'ed region is written to using write(2)
* both the data in the ARC and existing pages in the page cache
* are updated. For a read(2) data will be read first from the page
* cache then the ARC if needed. Neither a write(2) or read(2) will
* will ever result in new pages being added to the page cache.
*
* New pages are added to the page cache only via .readpage() which
* is called when the vfs needs to read a page off disk to back the
* virtual memory region. These pages may be modified without
* notifying the ARC and will be written out periodically via
* .writepage(). This will occur due to either a sync or the usual
* page aging behavior. Note because a read(2) of a mmap'ed file
* will always check the page cache first even when the ARC is out
* of date correct data will still be returned.
*
* While this implementation ensures correct behavior it does have
* have some drawbacks. The most obvious of which is that it
* increases the required memory footprint when access mmap'ed
* files. It also adds additional complexity to the code keeping
* both caches synchronized.
*
* Longer term it may be possible to cleanly resolve this wart by
* mapping page cache pages directly on to the ARC buffers. The
* Linux address space operations are flexible enough to allow
* selection of which pages back a particular index. The trick
* would be working out the details of which subsystem is in
* charge, the ARC, the page cache, or both. It may also prove
* helpful to move the ARC buffers to a scatter-gather lists
* rather than a vmalloc'ed region.
*/
static int
zpl_mmap(struct file *filp, struct vm_area_struct *vma)
{
struct inode *ip = filp->f_mapping->host;
znode_t *zp = ITOZ(ip);
int error;
error = -zfs_map(ip, vma->vm_pgoff, (caddr_t *)vma->vm_start,
(size_t)(vma->vm_end - vma->vm_start), vma->vm_flags);
if (error)
return (error);
error = generic_file_mmap(filp, vma);
if (error)
return (error);
mutex_enter(&zp->z_lock);
zp->z_is_mapped = 1;
mutex_exit(&zp->z_lock);
return (error);
}
static struct page **
pages_vector_from_list(struct list_head *pages, unsigned nr_pages)
{
struct page **pl;
struct page *t;
unsigned page_idx;
pl = kmalloc(sizeof(*pl) * nr_pages, GFP_NOFS);
if (!pl)
return ERR_PTR(-ENOMEM);
page_idx = 0;
list_for_each_entry_reverse(t, pages, lru) {
pl[page_idx] = t;
page_idx++;
}
return pl;
}
static int
zpl_readpages(struct file *file, struct address_space *mapping,
struct list_head *pages, unsigned nr_pages)
{
struct inode *ip;
struct page **pl;
struct page *p, *n;
int error;
ip = mapping->host;
pl = pages_vector_from_list(pages, nr_pages);
if (IS_ERR(pl))
return PTR_ERR(pl);
error = -zfs_getpage(ip, pl, nr_pages);
if (error)
goto error;
list_for_each_entry_safe_reverse(p, n, pages, lru) {
list_del(&p->lru);
flush_dcache_page(p);
SetPageUptodate(p);
unlock_page(p);
page_cache_release(p);
}
error:
kfree(pl);
return error;
}
/*
* Populate a page with data for the Linux page cache. This function is
* only used to support mmap(2). There will be an identical copy of the
* data in the ARC which is kept up to date via .write() and .writepage().
*
* Current this function relies on zpl_read_common() and the O_DIRECT
* flag to read in a page. This works but the more correct way is to
* update zfs_fillpage() to be Linux friendly and use that interface.
*/
static int
zpl_readpage(struct file *filp, struct page *pp)
{
struct inode *ip;
struct page *pl[1];
int error = 0;
ASSERT(PageLocked(pp));
ip = pp->mapping->host;
pl[0] = pp;
error = -zfs_getpage(ip, pl, 1);
if (error) {
SetPageError(pp);
ClearPageUptodate(pp);
} else {
ClearPageError(pp);
SetPageUptodate(pp);
flush_dcache_page(pp);
}
unlock_page(pp);
return error;
}
int
zpl_putpage(struct page *pp, struct writeback_control *wbc, void *data)
{
struct address_space *mapping = data;
ASSERT(PageLocked(pp));
ASSERT(!PageWriteback(pp));
/*
* Disable the normal reclaim path for zpl_putpage(). This
* ensures that all memory allocations under this call path
* will never enter direct reclaim. If this were to happen
* the VM might try to write out additional pages by calling
* zpl_putpage() again resulting in a deadlock.
*/
current->flags |= PF_MEMALLOC;
(void) zfs_putpage(mapping->host, pp, wbc);
current->flags &= ~PF_MEMALLOC;
return (0);
}
static int
zpl_writepages(struct address_space *mapping, struct writeback_control *wbc)
{
return write_cache_pages(mapping, wbc, zpl_putpage, mapping);
}
/*
* Write out dirty pages to the ARC, this function is only required to
* support mmap(2). Mapped pages may be dirtied by memory operations
* which never call .write(). These dirty pages are kept in sync with
* the ARC buffers via this hook.
*/
static int
zpl_writepage(struct page *pp, struct writeback_control *wbc)
{
return zpl_putpage(pp, wbc, pp->mapping);
}
const struct address_space_operations zpl_address_space_operations = {
.readpages = zpl_readpages,
.readpage = zpl_readpage,
.writepage = zpl_writepage,
.writepages = zpl_writepages,
};
const struct file_operations zpl_file_operations = {
.open = zpl_open,
.release = zpl_release,
.llseek = generic_file_llseek,
.read = zpl_read,
.write = zpl_write,
.readdir = zpl_readdir,
.mmap = zpl_mmap,
.fsync = zpl_fsync,
};
const struct file_operations zpl_dir_file_operations = {
.llseek = generic_file_llseek,
.read = generic_read_dir,
.readdir = zpl_readdir,
.fsync = zpl_fsync,
};