Re: [PATCH/RFC] Is it OK for 'read' to return nuls for a file that never had nuls in it?

[Neil Brown <neilb@suse.de>]

On Tuesday May 29, nickpiggin@yahoo.com.au wrote:
> > 
> > Questions:
> >   - Is this a problem, and should it be fixed (I think "yes").
> 
> I think you are right.

That's encouraging - thanks.

> 
> >   - Is the patch appropriate, and does it have no negative
> >     consequences?.
> >     (Obviously some comments should be tidied up to reflect the new
> >     reality).
> 
> Would it be better (and closer to following the existing logic) if
> we sampled i_size before testing each page for uptodateness? It might
> also cost a little less in the fastpath case of finding an uptodate
> page.

Uhm. no.

It doesn't address the issue which is that we need to check i_size
*after* we have got hold of the page and believe it to be uptodate.

And just to check, I race the test with your patch and I get a nice
little row of "!!!!!" - it is finding zeros still.

But we could possibly remove the early checks on end_index, that
makes sense (I've also moved the comment).

Thanks,
NeilBrown


Signed-off-by: Neil Brown <neilb@suse.de>

### Diffstat output
 ./mm/filemap.c |   72 ++++++++++++++++++++++-----------------------------------
 1 file changed, 28 insertions(+), 44 deletions(-)

diff .prev/mm/filemap.c ./mm/filemap.c
--- .prev/mm/filemap.c	2007-05-29 16:45:26.000000000 +1000
+++ ./mm/filemap.c	2007-05-29 16:58:23.000000000 +1000
@@ -875,13 +875,11 @@ void do_generic_mapping_read(struct addr
 {
 	struct inode *inode = mapping->host;
 	unsigned long index;
-	unsigned long end_index;
 	unsigned long offset;
 	unsigned long last_index;
 	unsigned long next_index;
 	unsigned long prev_index;
 	unsigned int prev_offset;
-	loff_t isize;
 	struct page *cached_page;
 	int error;
 	struct file_ra_state ra = *_ra;
@@ -894,27 +892,12 @@ void do_generic_mapping_read(struct addr
 	last_index = (*ppos + desc->count + PAGE_CACHE_SIZE-1) >> PAGE_CACHE_SHIFT;
 	offset = *ppos & ~PAGE_CACHE_MASK;
 
-	isize = i_size_read(inode);
-	if (!isize)
-		goto out;
-
-	end_index = (isize - 1) >> PAGE_CACHE_SHIFT;
 	for (;;) {
 		struct page *page;
+		unsigned long end_index;
+		loff_t isize;
 		unsigned long nr, ret;
 
-		/* nr is the maximum number of bytes to copy from this page */
-		nr = PAGE_CACHE_SIZE;
-		if (index >= end_index) {
-			if (index > end_index)
-				goto out;
-			nr = ((isize - 1) & ~PAGE_CACHE_MASK) + 1;
-			if (nr <= offset) {
-				goto out;
-			}
-		}
-		nr = nr - offset;
-
 		cond_resched();
 		if (index == next_index)
 			next_index = page_cache_readahead(mapping, &ra, filp,
@@ -929,6 +912,32 @@ find_page:
 		if (!PageUptodate(page))
 			goto page_not_up_to_date;
 page_ok:
+		/*
+		 * i_size must be checked after we know the page is Uptodate.
+		 *
+		 * Checking i_size after the check allows us to calculate
+		 * the correct value for "nr", which means the zero-filled
+		 * part of the page is not copied back to userspace (unless
+		 * another truncate extends the file - this is desired though).
+		 */
+
+		isize = i_size_read(inode);
+		end_index = (isize - 1) >> PAGE_CACHE_SHIFT;
+		if (unlikely(!isize || index > end_index)) {
+			page_cache_release(page);
+			goto out;
+		}
+
+		/* nr is the maximum number of bytes to copy from this page */
+		nr = PAGE_CACHE_SIZE;
+		if (index == end_index) {
+			nr = ((isize - 1) & ~PAGE_CACHE_MASK) + 1;
+			if (nr <= offset) {
+				page_cache_release(page);
+				goto out;
+			}
+		}
+		nr = nr - offset;
 
 		/* If users can be writing to this page using arbitrary
 		 * virtual addresses, take care about potential aliasing
@@ -1015,31 +1024,6 @@ readpage:
 			unlock_page(page);
 		}
 
-		/*
-		 * i_size must be checked after we have done ->readpage.
-		 *
-		 * Checking i_size after the readpage allows us to calculate
-		 * the correct value for "nr", which means the zero-filled
-		 * part of the page is not copied back to userspace (unless
-		 * another truncate extends the file - this is desired though).
-		 */
-		isize = i_size_read(inode);
-		end_index = (isize - 1) >> PAGE_CACHE_SHIFT;
-		if (unlikely(!isize || index > end_index)) {
-			page_cache_release(page);
-			goto out;
-		}
-
-		/* nr is the maximum number of bytes to copy from this page */
-		nr = PAGE_CACHE_SIZE;
-		if (index == end_index) {
-			nr = ((isize - 1) & ~PAGE_CACHE_MASK) + 1;
-			if (nr <= offset) {
-				page_cache_release(page);
-				goto out;
-			}
-		}
-		nr = nr - offset;
 		goto page_ok;
 
 readpage_error:
-
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