2019-03-09 15:56:31

by Sean Fu

Subject: [PATCH] xfs: Use kmem_zalloc for bp->b_pages.

Change the allocation of bp->b_pages to use kmem_zalloc instead of
kmem_alloc.
Remove unnecessary memset for bp->b_pages.

This reduces text size by 42 bytes.
Before:
text data bss dec hex filename
23335 588 8 23931 5d7b ./fs/xfs/xfs_buf.o
After:
text data bss dec hex filename
23293 588 8 23889 5d51 ./fs/xfs/xfs_buf.o

Signed-off-by: Sean Fu <[email protected]>
---
fs/xfs/xfs_buf.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
index 4f5f2ff3f70f..be4f740b97c1 100644
--- a/fs/xfs/xfs_buf.c
+++ b/fs/xfs/xfs_buf.c
@@ -289,12 +289,11 @@ _xfs_buf_get_pages(
if (page_count <= XB_PAGES) {
bp->b_pages = bp->b_page_array;
} else {
- bp->b_pages = kmem_alloc(sizeof(struct page *) *
+ bp->b_pages = kmem_zalloc(sizeof(struct page *) *
page_count, KM_NOFS);
if (bp->b_pages == NULL)
return -ENOMEM;
}
- memset(bp->b_pages, 0, sizeof(struct page *) * page_count);
}
return 0;
}
--
2.16.4

2019-03-09 17:34:07

by Darrick J. Wong

Subject: Re: [PATCH] xfs: Use kmem_zalloc for bp->b_pages.

On Sat, Mar 09, 2019 at 11:36:36PM +0800, Sean Fu wrote:
> Change the allocation of bp->b_pages to use kmem_zalloc instead of
> kmem_alloc.
> Remove unnecessary memset for bp->b_pages.
>
> This reduces text size by 42 bytes.
> Before:
> text data bss dec hex filename
> 23335 588 8 23931 5d7b ./fs/xfs/xfs_buf.o
> After:
> text data bss dec hex filename
> 23293 588 8 23889 5d51 ./fs/xfs/xfs_buf.o
>
> Signed-off-by: Sean Fu <[email protected]>
> ---
> fs/xfs/xfs_buf.c | 3 +--
> 1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> index 4f5f2ff3f70f..be4f740b97c1 100644
> --- a/fs/xfs/xfs_buf.c
> +++ b/fs/xfs/xfs_buf.c
> @@ -289,12 +289,11 @@ _xfs_buf_get_pages(
> if (page_count <= XB_PAGES) {
> bp->b_pages = bp->b_page_array;
> } else {
> - bp->b_pages = kmem_alloc(sizeof(struct page *) *
> + bp->b_pages = kmem_zalloc(sizeof(struct page *) *
> page_count, KM_NOFS);
> if (bp->b_pages == NULL)
> return -ENOMEM;
> }
> - memset(bp->b_pages, 0, sizeof(struct page *) * page_count);

Does this leave b_pages uninitialized in the page_count <= XB_PAGES
case?

--D

> }
> return 0;
> }
> --
> 2.16.4
>

2019-03-10 05:10:02

by Sean Fu

Subject: Re: [PATCH] xfs: Use kmem_zalloc for bp->b_pages.

On Sat, Mar 09, 2019 at 09:32:30AM -0800, Darrick J. Wong wrote:
> On Sat, Mar 09, 2019 at 11:36:36PM +0800, Sean Fu wrote:
> > Change the allocation of bp->b_pages to use kmem_zalloc instead of
> > kmem_alloc.
> > Remove unnecessary memset for bp->b_pages.
> >
> > This reduces text size by 42 bytes.
> > Before:
> > text data bss dec hex filename
> > 23335 588 8 23931 5d7b ./fs/xfs/xfs_buf.o
> > After:
> > text data bss dec hex filename
> > 23293 588 8 23889 5d51 ./fs/xfs/xfs_buf.o
> >
> > Signed-off-by: Sean Fu <[email protected]>
> > ---
> > fs/xfs/xfs_buf.c | 3 +--
> > 1 file changed, 1 insertion(+), 2 deletions(-)
> >
> > diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> > index 4f5f2ff3f70f..be4f740b97c1 100644
> > --- a/fs/xfs/xfs_buf.c
> > +++ b/fs/xfs/xfs_buf.c
> > @@ -289,12 +289,11 @@ _xfs_buf_get_pages(
> > if (page_count <= XB_PAGES) {
> > bp->b_pages = bp->b_page_array;
> > } else {
> > - bp->b_pages = kmem_alloc(sizeof(struct page *) *
> > + bp->b_pages = kmem_zalloc(sizeof(struct page *) *
> > page_count, KM_NOFS);
> > if (bp->b_pages == NULL)
> > return -ENOMEM;
> > }
> > - memset(bp->b_pages, 0, sizeof(struct page *) * page_count);
>
> Does this leave b_pages uninitialized in the page_count <= XB_PAGES
> case?
bp is allocated by kmem_zone_zalloc, But i will take a deep look at xfs_buf_associate_memory.
>
> --D
>
> > }
> > return 0;
> > }
> > --
> > 2.16.4
> >

2019-03-10 22:11:52

by Dave Chinner

Subject: Re: [PATCH] xfs: Use kmem_zalloc for bp->b_pages.

On Sun, Mar 10, 2019 at 01:07:32PM +0800, Sean Fu wrote:
> On Sat, Mar 09, 2019 at 09:32:30AM -0800, Darrick J. Wong wrote:
> > On Sat, Mar 09, 2019 at 11:36:36PM +0800, Sean Fu wrote:
> > > Change the allocation of bp->b_pages to use kmem_zalloc instead of
> > > kmem_alloc.
> > > Remove unnecessary memset for bp->b_pages.
> > >
> > > This reduces text size by 42 bytes.
> > > Before:
> > > text data bss dec hex filename
> > > 23335 588 8 23931 5d7b ./fs/xfs/xfs_buf.o
> > > After:
> > > text data bss dec hex filename
> > > 23293 588 8 23889 5d51 ./fs/xfs/xfs_buf.o
> > >
> > > Signed-off-by: Sean Fu <[email protected]>
> > > ---
> > > fs/xfs/xfs_buf.c | 3 +--
> > > 1 file changed, 1 insertion(+), 2 deletions(-)
> > >
> > > diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> > > index 4f5f2ff3f70f..be4f740b97c1 100644
> > > --- a/fs/xfs/xfs_buf.c
> > > +++ b/fs/xfs/xfs_buf.c
> > > @@ -289,12 +289,11 @@ _xfs_buf_get_pages(
> > > if (page_count <= XB_PAGES) {
> > > bp->b_pages = bp->b_page_array;
> > > } else {
> > > - bp->b_pages = kmem_alloc(sizeof(struct page *) *
> > > + bp->b_pages = kmem_zalloc(sizeof(struct page *) *
> > > page_count, KM_NOFS);
> > > if (bp->b_pages == NULL)
> > > return -ENOMEM;
> > > }
> > > - memset(bp->b_pages, 0, sizeof(struct page *) * page_count);
> >
> > Does this leave b_pages uninitialized in the page_count <= XB_PAGES
> > case?
> bp is allocated by kmem_zone_zalloc, But i will take a deep look at xfs_buf_associate_memory.

We can (and do) reuse buffers with different memory buffers via
xfs_buf_associate_memory() (e.g. see the split log buffer code in
xlog_sync()].

I'd prefer that we leave the explicit memset here for this re-use
case - leaving the page pointer array with stale page pointers (even
though beyond b_page_count) means any page array overrun turns into
a memory corruption (via access of a stale page) rather than a NULL
pointer dereference and immediate fail.

IMO, a small decrease in binary size is not worth changing the
failure behaviour from a definite, immediate fail to a random memory
corruption failure some time in the future...

Cheers,

Dave.
--
Dave Chinner
[email protected]