From: Paolo 'Blaisorblade' Giarrusso <[email protected]>
Even now, we'll sometimes take the write lock. So, in that case, we could
downgrade it; after a tiny bit of thought, I've choosen doing that when we'll
either do any I/O or we'll alter a lot of PTEs. About how much "a lot" is,
I've copied the values from this code in mm/memory.c:
#ifdef CONFIG_PREEMPT
# define ZAP_BLOCK_SIZE (8 * PAGE_SIZE)
#else
/* No preempt: go for improved straight-line efficiency */
# define ZAP_BLOCK_SIZE (1024 * PAGE_SIZE)
#endif
I'm not sure about the trade-offs - we used to have a down_write, now we have
a down_read() and a possible up_read()down_write(), and with this patch, the
fast-path still takes only down_read, but the slow path will do down_read(),
down_write(), downgrade_write(). This will increase the number of atomic
operation but increase concurrency wrt mmap and similar operations - I don't
know how much contention there is on that lock.
Also, drop a bust comment: we cannot clear VM_NONLINEAR simply because code
elsewhere is going to use it. At the very least, madvise_dontneed() relies on
that flag being set (remaining non-linear truncation read the mapping
list), but the list is probably longer and going to increase in the next
patches of this series.
Just in case this wasn't clear: this patch is not strictly related to
protection support, I was just too lazy to move it up in the hierarchy.
Signed-off-by: Paolo 'Blaisorblade' Giarrusso <[email protected]>
---
linux-2.6.git-paolo/mm/fremap.c | 18 +++++++++++++-----
1 files changed, 13 insertions(+), 5 deletions(-)
diff -puN mm/fremap.c~rfp-downgrade-lock mm/fremap.c
--- linux-2.6.git/mm/fremap.c~rfp-downgrade-lock 2005-08-11 23:04:39.000000000 +0200
+++ linux-2.6.git-paolo/mm/fremap.c 2005-08-11 23:04:39.000000000 +0200
@@ -152,6 +152,13 @@ err_unlock:
}
+#ifdef CONFIG_PREEMPT
+# define INSTALL_SIZE (8 * PAGE_SIZE)
+#else
+/* No preempt: go for improved straight-line efficiency */
+# define INSTALL_SIZE (1024 * PAGE_SIZE)
+#endif
+
/***
* sys_remap_file_pages - remap arbitrary pages of a shared backing store
* file within an existing vma.
@@ -266,14 +273,15 @@ retry:
}
}
+ /* Do NOT hold the write lock while doing any I/O, nor when
+ * iterating over too many PTEs. Values might need tuning. */
+ if (has_write_lock && (!(flags & MAP_NONBLOCK) || size > INSTALL_SIZE)) {
+ downgrade_write(&mm->mmap_sem);
+ has_write_lock = 0;
+ }
err = vma->vm_ops->populate(vma, start, size, pgprot, pgoff,
flags & MAP_NONBLOCK);
- /*
- * We can't clear VM_NONLINEAR because we'd have to do
- * it after ->populate completes, and that would prevent
- * downgrading the lock. (Locks can't be upgraded).
- */
}
out_unlock:
_
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
| |
 |