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Subject[RFC v2] block integrity: Stabilize(?) pages during writeback
On Wed, Apr 13, 2011 at 05:48:48PM -0700, Mingming Cao wrote:
> On Mon, 2011-04-11 at 20:57 -0400, Christoph Hellwig wrote:
> > On Mon, Apr 11, 2011 at 05:46:52PM -0700, Mingming Cao wrote:
> > > Oh, right. Currently ext4_page_mkwrite drops the page lock before
> > > calling it's dirty the page (by write_begin() and write_end(). I
> > > suspect regrab the lock() after write_end() (with your proposed change)
> > > and returning with locked still leave the dirty by ext4_page_mkwrite
> > > unlocked. We probably should to keep the page locked the page during
> > > the entire ext4_page_mkwrite() call. Any reason to drop the page lock()
> > > before calling aops->write_begin()?
> >
> > write_begin takes the page lock by itself. That's one of the reasons why
> > block_page_mkwrite doesn't use plain ->write_begin / write_end, the
> > other beeing that we already get a page passed to use, so there's no
> > need to do the pagecache lookup or allocation done by
> > grab_cache_page_write_begin.
> >
> > The best thing would be to completely drop ext4's current version
> > of page_mkwrite and start out with a copy of block_page_mkwrite which
> > has the journalling calls added back into it.
>
> The problem is the locking order, we can't hold page lock then start the
> journal lock. Kjournald will need to hold the journal lock first, then
> commit, commit may need to callback writepages, which need to hold the
> page lock.
>
>
> I looked at ext3, in that case, ext3 even don't have ext3_page_mkwrite()
> to do the stable page yet. It requires some block reservation/delayed
> allocation for filling holes in mmaped IO case. Jan tried that before I
> don't think the proposal get far.
>
>
> Now looking back ext4_page_mkwrite(), it calls write_begin(), as long as
> we do wait in write_begin() things would be fine, right? It seems
> Darrick already did that wait per Dave Chinner's suggestion when grab
> the page and lock that page. But still a puzzle to me why that's not
> sufficient.

Hi everyone,

I've finally managed to get together a patch that seems to provide stable pages
during writeback, or at least gets us to the point that after several days of
running tests I don't see DIF checksum errors anymore. :)

The last two pieces to go into this puzzle were (a) bio_integrity_prep needs to
walk the process tree to find all userland ptes that map to a particular memory
page and revoke write access, and (b) ext4_page_mkwrite should always lock (and
wait on) a page before returning. There are still some huge warts with the
patch; in particular, the x86-specific set_memory_r[ow] needs to go away, which
is what I was trying to do with the #ifdef USE_X86 case. Also, there are
probably more wait_for_page_writeback()s than are really necessary.

Going forward, however, it might be easier to take all those waits out and
simply copy-migrate the page to another location when the page fault handler
sees the write-during-io case, so I'll look into that tomorrow. I'm also not
particularly sure what happens when huge pages get involved.

Debug-quality patch follows, before my kernel-build disks crash again.

--D
---

diff --git a/fs/bio-integrity.c b/fs/bio-integrity.c
index 9c5e6b2..8031bd7 100644
--- a/fs/bio-integrity.c
+++ b/fs/bio-integrity.c
@@ -25,6 +25,7 @@
#include <linux/bio.h>
#include <linux/workqueue.h>
#include <linux/slab.h>
+#include <asm/tlbflush.h>

struct integrity_slab {
struct kmem_cache *slab;
@@ -382,6 +383,110 @@ static inline unsigned short blk_integrity_tuple_size(struct blk_integrity *bi)
return 0;
}

+#define USE_X86
+
+#ifdef USE_X86
+static int set_page_ro(struct page *page)
+{
+ int set_memory_ro(unsigned long addr, int numpages);
+ unsigned long addr = (unsigned long)page_address(page);
+ if (PageHighMem(page))
+ return 0;
+ return set_memory_ro(addr, 1);
+}
+
+static int set_page_rw(struct page *page)
+{
+ int set_memory_rw(unsigned long addr, int numpages);
+ unsigned long addr = (unsigned long)page_address(page);
+ if (PageHighMem(page))
+ return 0;
+ return set_memory_rw(addr, 1);
+}
+#endif
+
+#include <linux/rmap.h>
+static void set_bio_page_access(struct mm_struct *mm, struct bio *bio, int rw)
+{
+ struct bio_vec *from;
+ int i;
+
+ __bio_for_each_segment(from, bio, i, 0) {
+ struct vm_area_struct *vma;
+ int modified = 0;
+
+ for (vma = mm->mmap; vma; vma = vma->vm_next) {
+ unsigned long address;
+ pte_t *pte;
+ spinlock_t *ptl;
+
+ // grab pte, change to ro?
+ address = page_address_in_vma(from->bv_page, vma);
+ if (address == -EFAULT) /* out of vma range */
+ continue;
+ pte = page_check_address(from->bv_page, vma->vm_mm, address, &ptl, 1);
+ if (!pte) /* the page is not in this mm */
+ continue;
+ modified = 1;
+ if (rw) {
+ pte_t old_pte = *pte;
+ set_pte_at(mm, address, pte, pte_mkwrite(old_pte));
+ } else
+ ptep_set_wrprotect(mm, address, pte);
+ pte_unmap_unlock(pte, ptl);
+ }
+ if (modified)
+ flush_tlb_mm(mm);
+ }
+}
+
+static void set_bio_page_rw(struct mm_struct *mm, struct bio *bio)
+{
+ set_bio_page_access(mm, bio, 1);
+}
+
+static void set_bio_page_ro(struct mm_struct *mm, struct bio *bio)
+{
+ set_bio_page_access(mm, bio, 0);
+}
+
+#ifdef USE_X86
+static void bio_integrity_write_fn(struct work_struct *work)
+{
+ struct bio_integrity_payload *bip =
+ container_of(work, struct bio_integrity_payload, bip_work);
+ struct bio *bio = bip->bip_bio;
+ struct bio_vec *from;
+ int i;
+
+ __bio_for_each_segment(from, bio, i, 0) {
+ set_page_rw(from->bv_page);
+ }
+
+ /* Restore original bio completion handler */
+ bio->bi_end_io = bip->bip_end_io;
+ bio_endio(bio, bip->bip_error);
+}
+#endif
+
+static void bio_integrity_endio_write(struct bio *bio, int error)
+{
+ struct bio_integrity_payload *bip = bio->bi_integrity;
+
+ BUG_ON(bip->bip_bio != bio);
+
+#ifndef USE_X86
+ set_bio_page_rw(&init_mm, bio);
+ bio->bi_end_io = bip->bip_end_io;
+ bio_endio(bio, bip->bip_error);
+ return;
+#else
+ bip->bip_error = error;
+ INIT_WORK(&bip->bip_work, bio_integrity_write_fn);
+ queue_work(kintegrityd_wq, &bip->bip_work);
+#endif
+}
+
/**
* bio_integrity_prep - Prepare bio for integrity I/O
* @bio: bio to prepare
@@ -468,8 +573,42 @@ int bio_integrity_prep(struct bio *bio)
}

/* Auto-generate integrity metadata if this is a write */
- if (bio_data_dir(bio) == WRITE)
+ if (bio_data_dir(bio) == WRITE) {
+ struct task_struct *p;
+ struct mm_struct *mm;
+
+ bip->bip_end_io = bio->bi_end_io;
+ bio->bi_end_io = bio_integrity_endio_write;
+
+#ifndef USE_X86
+// kmap_flush_unused();
+// vm_unmap_aliases();
+ set_bio_page_ro(&init_mm, bio);
+ flush_tlb_all();
+#else
+ {
+ struct bio_vec *from;
+ int i;
+
+ __bio_for_each_segment(from, bio, i, 0) {
+ set_page_ro(from->bv_page);
+ }
+ }
+#endif
+
+ for_each_process(p) {
+ task_lock(p);
+ mm = p->mm;
+ if (!mm) {
+ task_unlock(p);
+ continue;
+ }
+
+ set_bio_page_ro(mm, bio);
+ task_unlock(p);
+ }
bio_integrity_generate(bio);
+ }

return 0;
}
diff --git a/fs/buffer.c b/fs/buffer.c
index a08bb8e..02156c5 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -376,8 +376,10 @@ void end_buffer_async_write(struct buffer_head *bh, int uptodate)
if (!quiet_error(bh)) {
buffer_io_error(bh);
printk(KERN_WARNING "lost page write due to "
- "I/O error on %s\n",
- bdevname(bh->b_bdev, b));
+ "I/O error on %s, flags=%lx\n",
+ bdevname(bh->b_bdev, b), page->flags);
+if (page->mapping && page->mapping->host)
+printk(KERN_ERR "mode=%x inode=%d dev=%d\n", page->mapping->host->i_mode, page->mapping->host->i_ino, (page->mapping->host->i_rdev));
}
set_bit(AS_EIO, &page->mapping->flags);
set_buffer_write_io_error(bh);
@@ -2357,6 +2359,8 @@ block_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf,
else
end = PAGE_CACHE_SIZE;

+ WARN_ON(!PageLocked(page));
+ wait_on_page_writeback(page);
ret = __block_write_begin(page, 0, end, get_block);
if (!ret)
ret = block_commit_write(page, 0, end);
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index f2fa5e8..228b4aa 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -2667,6 +2667,8 @@ static int ext4_writepage(struct page *page,
*/
goto redirty_page;
}
+ WARN_ON(!PageLocked(page));
+ wait_on_page_writeback(page);
if (commit_write)
/* now mark the buffer_heads as dirty and uptodate */
block_commit_write(page, 0, len);
@@ -2779,7 +2781,8 @@ static int write_cache_pages_da(struct address_space *mapping,
}

lock_page(page);
-
+ if (PageWriteback(page))
+ wait_on_page_writeback(page);
/*
* If the page is no longer dirty, or its
* mapping no longer corresponds to inode we
@@ -5811,15 +5814,21 @@ int ext4_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
goto out_unlock;
}
ret = 0;
- if (PageMappedToDisk(page))
- goto out_unlock;
+ lock_page(page);
+ if (PageWriteback(page))
+ wait_on_page_writeback(page);
+ if (PageMappedToDisk(page)) {
+ up_read(&inode->i_alloc_sem);
+ return VM_FAULT_LOCKED;
+ }

+ /* this one seems to handle mmap */
+ wait_on_page_writeback(page);
if (page->index == size >> PAGE_CACHE_SHIFT)
len = size & ~PAGE_CACHE_MASK;
else
len = PAGE_CACHE_SIZE;

- lock_page(page);
/*
* return if we have all the buffers mapped. This avoid
* the need to call write_begin/write_end which does a
@@ -5829,8 +5838,8 @@ int ext4_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
if (page_has_buffers(page)) {
if (!walk_page_buffers(NULL, page_buffers(page), 0, len, NULL,
ext4_bh_unmapped)) {
- unlock_page(page);
- goto out_unlock;
+ up_read(&inode->i_alloc_sem);
+ return VM_FAULT_LOCKED;
}
}
unlock_page(page);
@@ -5850,6 +5859,17 @@ int ext4_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
if (ret < 0)
goto out_unlock;
ret = 0;
+
+ /*
+ * write_begin/end might have created a dirty page and someone
+ * could wander in and start the IO. Make sure that hasn't
+ * happened.
+ */
+ lock_page(page);
+ if (PageWriteback(page))
+ wait_on_page_writeback(page);
+ up_read(&inode->i_alloc_sem);
+ return VM_FAULT_LOCKED;
out_unlock:
if (ret)
ret = VM_FAULT_SIGBUS;
diff --git a/include/linux/bio.h b/include/linux/bio.h
index ce33e68..ea36e89 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -180,6 +180,7 @@ struct bio_integrity_payload {
unsigned short bip_vcnt; /* # of integrity bio_vecs */
unsigned short bip_idx; /* current bip_vec index */

+ int bip_error; /* bio completion status? */
struct work_struct bip_work; /* I/O completion */
struct bio_vec bip_vec[0]; /* embedded bvec array */
};
diff --git a/mm/filemap.c b/mm/filemap.c
index c641edf..3ed13a3 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2277,8 +2277,9 @@ EXPORT_SYMBOL(generic_file_direct_write);
* Find or create a page at the given pagecache position. Return the locked
* page. This function is specifically for buffered writes.
*/
-struct page *grab_cache_page_write_begin(struct address_space *mapping,
- pgoff_t index, unsigned flags)
+static struct page *
+__grab_cache_page_write_begin(struct address_space *mapping, pgoff_t index,
+ unsigned flags)
{
int status;
struct page *page;
@@ -2303,6 +2304,20 @@ repeat:
}
return page;
}
+
+struct page *grab_cache_page_write_begin(struct address_space *mapping,
+ pgoff_t index, unsigned flags)
+{
+ struct page *p;
+
+ p = __grab_cache_page_write_begin(mapping, index, flags);
+ if (p) {
+ WARN_ON(!PageLocked(p));
+ wait_on_page_writeback(p);
+ }
+
+ return p;
+}
EXPORT_SYMBOL(grab_cache_page_write_begin);

static ssize_t generic_perform_write(struct file *file,
diff --git a/mm/memory.c b/mm/memory.c
index ce22a25..9d3dc5d 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3156,6 +3156,9 @@ static int __do_fault(struct mm_struct *mm, struct vm_area_struct *vma,
} else
VM_BUG_ON(!PageLocked(page));
page_mkwrite = 1;
+ } else {
+ WARN_ON(!PageLocked(page));
+ wait_on_page_writeback(page);
}
}


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