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Date: Thu, 24 Jan 2008 01:05:04 +0100
From: Miklos Szeredi <miklos@...redi.hu>
To: torvalds@...ux-foundation.org
CC: miklos@...redi.hu, a.p.zijlstra@...llo.nl, salikhmetov@...il.com,
linux-mm@...ck.org, jakob@...hought.net,
linux-kernel@...r.kernel.org, valdis.kletnieks@...edu,
riel@...hat.com, ksm@...dk, staubach@...hat.com,
jesper.juhl@...il.com, akpm@...ux-foundation.org,
protasnb@...il.com, r.e.wolff@...wizard.nl,
hidave.darkstar@...il.com, hch@...radead.org
Subject: Re: [PATCH -v8 3/4] Enable the MS_ASYNC functionality in
sys_msync()
> > How about doing it in a separate pass, similarly to
> > wait_on_page_writeback()? Just instead of waiting, clean the page
> > tables for writeback pages.
>
> That sounds like a good idea, but it doesn't work.
>
> The thing is, we need to hold the page-table lock over the whole sequence
> of
>
> if (page_mkclean(page))
> set_page_dirty(page);
> if (TestClearPageDirty(page))
> ..
>
> and there's a big comment about why in clear_page_dirty_for_io().
>
> So if you split it up, so that the first phase is that
>
> if (page_mkclean(page))
> set_page_dirty(page);
>
> and the second phase is the one that just does a
>
> if (TestClearPageDirty(page))
> writeback(..)
>
> and having dropped the page lock in between, then you lose: because
> another thread migth have faulted in and re-dirtied the page table entry,
> and you MUST NOT do that "TestClearPageDirty()" in that case!
>
> That dirty bit handling is really really important, and it's sadly also
> really really easy to get wrong (usually in ways that are hard to even
> notice: things still work 99% of the time, and you might just be leaking
> memory slowly, and fsync/msync() might not write back memory mapped data
> to disk at all etc).
OK.
But I still think this approach should work. Untested, might eat your
children, so please don't apply to any kernel.
Miklos
Index: linux/mm/msync.c
===================================================================
--- linux.orig/mm/msync.c 2008-01-24 00:18:31.000000000 +0100
+++ linux/mm/msync.c 2008-01-24 00:50:37.000000000 +0100
@@ -10,9 +10,91 @@
#include <linux/fs.h>
#include <linux/mm.h>
#include <linux/mman.h>
+#include <linux/pagemap.h>
#include <linux/file.h>
#include <linux/syscalls.h>
#include <linux/sched.h>
+#include <linux/pagevec.h>
+#include <linux/rmap.h>
+#include <linux/backing-dev.h>
+
+static void mkclean_writeback_pages(struct address_space *mapping,
+ pgoff_t start, pgoff_t end)
+{
+ struct pagevec pvec;
+ pgoff_t index;
+
+ if (!mapping_cap_account_dirty(mapping))
+ return;
+
+ if (end < start)
+ return;
+
+ pagevec_init(&pvec, 0);
+ index = start;
+ while (index <= end) {
+ unsigned i;
+ int nr_pages = min(end - index, (pgoff_t)PAGEVEC_SIZE-1) + 1;
+
+ nr_pages = pagevec_lookup_tag(&pvec, mapping, &index,
+ PAGECACHE_TAG_WRITEBACK,
+ nr_pages);
+ if (!nr_pages)
+ break;
+
+ for (i = 0; i < nr_pages; i++) {
+ struct page *page = pvec.pages[i];
+
+ /* until radix tree lookup accepts end_index */
+ if (page->index > end)
+ continue;
+
+ lock_page(page);
+ if (page_mkclean(page))
+ set_page_dirty(page);
+ unlock_page(page);
+ }
+ pagevec_release(&pvec);
+ cond_resched();
+ }
+}
+
+static int msync_range(struct file *file, loff_t start, unsigned long len, unsigned int sync)
+{
+ int ret;
+ struct address_space *mapping = file->f_mapping;
+ loff_t end = start + len - 1;
+ int sync_flags = SYNC_FILE_RANGE_WRITE;
+
+ if (sync) {
+ sync_flags |= SYNC_FILE_RANGE_WAIT_BEFORE;
+ } else {
+ /*
+ * For MS_ASYNC, don't wait for writback already in
+ * progress, but instead just clean the page tables.
+ */
+ mkclean_writeback_pages(mapping,
+ start >> PAGE_CACHE_SHIFT,
+ end >> PAGE_CACHE_SHIFT);
+ }
+
+ ret = do_sync_mapping_range(mapping, start, end, sync_flags);
+ if (ret || !sync)
+ return ret;
+
+ if (file->f_op && file->f_op->fsync) {
+ mutex_lock(&mapping->host->i_mutex);
+ ret = file->f_op->fsync(file, file->f_path.dentry, 0);
+ mutex_unlock(&mapping->host->i_mutex);
+
+ if (ret < 0)
+ return ret;
+ }
+
+ return wait_on_page_writeback_range(mapping,
+ start >> PAGE_CACHE_SHIFT,
+ end >> PAGE_CACHE_SHIFT);
+}
/*
* MS_SYNC syncs the entire file - including mappings.
@@ -77,18 +159,36 @@ asmlinkage long sys_msync(unsigned long
goto out_unlock;
}
file = vma->vm_file;
- start = vma->vm_end;
- if ((flags & MS_SYNC) && file &&
- (vma->vm_flags & VM_SHARED)) {
+
+ if (file && (vma->vm_flags & VM_SHARED) &&
+ (flags & (MS_SYNC | MS_ASYNC))) {
+ loff_t offset;
+ unsigned long len;
+
+ /*
+ * We need to do all of this before we release the mmap_sem,
+ * since "vma" isn't available after that.
+ */
+ offset = start - vma->vm_start;
+ offset += vma->vm_pgoff << PAGE_SHIFT;
+ len = end;
+ if (len > vma->vm_end)
+ len = vma->vm_end;
+ len -= start;
+
+ /* Update start here, since vm_end will be gone too.. */
+ start = vma->vm_end;
get_file(file);
up_read(&mm->mmap_sem);
- error = do_fsync(file, 0);
+
+ error = msync_range(file, offset, len, flags & MS_SYNC);
fput(file);
if (error || start >= end)
goto out;
down_read(&mm->mmap_sem);
vma = find_vma(mm, start);
} else {
+ start = vma->vm_end;
if (start >= end) {
error = 0;
goto out_unlock;
--
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