Re: [PATCHv2 1/3] fs/buffer.c: allocate buffer cache with user specific flag

Gioh Kim
2014-08-19T23:37:26Z

2014-08-19 오후 10:03, Jan Kara 쓴 글:
> Hello,
>
> On Tue 19-08-14 15:52:38, Gioh Kim wrote:
>> A buffer cache is allocated from movable area
>> because it is referred for a while and released soon.
>> But some filesystems are taking buffer cache for a long time
>> and it can disturb page migration.
>>
>> A new API should be introduced to allocate buffer cache
>> with user specific flag.
>> For instance if user set flag to zero, buffer cache is allocated from
>> non-movable area.
>>
>> Signed-off-by: Gioh Kim <gioh.kim@lge.com>
>> ---
>> fs/buffer.c | 52 +++++++++++++++++++++++++++++--------------
>> include/linux/buffer_head.h | 12 +++++++++-
>> 2 files changed, 46 insertions(+), 18 deletions(-)
>>
>> diff --git a/fs/buffer.c b/fs/buffer.c
>> index 8f05111..14f2f21 100644
>> --- a/fs/buffer.c
>> +++ b/fs/buffer.c
>> @@ -993,7 +993,7 @@ init_page_buffers(struct page *page, struct block_device *bdev,
>> */
>> static int
>> grow_dev_page(struct block_device *bdev, sector_t block,
>> - pgoff_t index, int size, int sizebits)
>> + pgoff_t index, int size, int sizebits, gfp_t gfp)
>> {
>> struct inode *inode = bdev->bd_inode;
>> struct page *page;
>> @@ -1002,10 +1002,10 @@ grow_dev_page(struct block_device *bdev, sector_t block,
>> int ret = 0; /* Will call free_more_memory() */
>> gfp_t gfp_mask;
>>
>> - gfp_mask = mapping_gfp_mask(inode->i_mapping) & ~__GFP_FS;
>> - gfp_mask |= __GFP_MOVABLE;
>> + gfp_mask = (mapping_gfp_mask(inode->i_mapping) & ~__GFP_FS) | gfp;
>> +
> Hum, it seems a bit misleading that the 'gfp' flags are just or-ed to
> mapping_gfp_mask(inode->i_mapping). Usually, passed gfp mask is just
> directly used. There are also interfaces like pagecache_get_page() which
> play more complex tricks with mapping_gfp_mask(). This would be yet another
> convention which I don't think is desirable. I know Andrew suggested what
> you wrote so I guess I have to settle this with him. Andrew?

I don't know mapping_gfp_mask(). I just add gfp at the original code.
Whould you tell me why it is undesirable?

>
>> /*
>> - * XXX: __getblk_slow() can not really deal with failure and
>> + * XXX: __getblk_gfp() can not really deal with failure and
>> * will endlessly loop on improvised global reclaim. Prefer
>> * looping in the allocator rather than here, at least that
>> * code knows what it's doing.
>> @@ -1058,7 +1058,7 @@ failed:
>> * that page was dirty, the buffers are set dirty also.
>> */
>> static int
>> -grow_buffers(struct block_device *bdev, sector_t block, int size)
>> +grow_buffers(struct block_device *bdev, sector_t block, int size, gfp_t gfp)
>> {
>> pgoff_t index;
>> int sizebits;
>> @@ -1085,11 +1085,12 @@ grow_buffers(struct block_device *bdev, sector_t block, int size)
>> }
>>
>> /* Create a page with the proper size buffers.. */
>> - return grow_dev_page(bdev, block, index, size, sizebits);
>> + return grow_dev_page(bdev, block, index, size, sizebits, gfp);
>> }
>>
>> -static struct buffer_head *
>> -__getblk_slow(struct block_device *bdev, sector_t block, int size)
>> +struct buffer_head *
>> +__getblk_gfp(struct block_device *bdev, sector_t block,
>> + unsigned size, gfp_t gfp)
>> {
>> /* Size must be multiple of hard sectorsize */
>> if (unlikely(size & (bdev_logical_block_size(bdev)-1) ||
>> @@ -1111,13 +1112,14 @@ __getblk_slow(struct block_device *bdev, sector_t block, int size)
>> if (bh)
>> return bh;
>>
>> - ret = grow_buffers(bdev, block, size);
>> + ret = grow_buffers(bdev, block, size, gfp);
>> if (ret < 0)
>> return NULL;
>> if (ret == 0)
>> free_more_memory();
>> }
>> }
>> +EXPORT_SYMBOL(__getblk_gfp);
>>
>> /*
>> * The relationship between dirty buffers and dirty pages:
>> @@ -1381,12 +1383,7 @@ EXPORT_SYMBOL(__find_get_block);
>> struct buffer_head *
>> __getblk(struct block_device *bdev, sector_t block, unsigned size)
>> {
>> - struct buffer_head *bh = __find_get_block(bdev, block, size);
>> -
>> - might_sleep();
>> - if (bh == NULL)
>> - bh = __getblk_slow(bdev, block, size);
>> - return bh;
>> + return __getblk_gfp(bdev, block, size, __GFP_MOVABLE);
>> }
>> EXPORT_SYMBOL(__getblk);
> Why did you remove the __find_get_block() call? That looks like a bug.
>
>> @@ -1410,18 +1407,39 @@ EXPORT_SYMBOL(__breadahead);
>> * @size: size (in bytes) to read
>> *
>> * Reads a specified block, and returns buffer head that contains it.
>> + * The page cache is allocated from movable area so that it can be migrated.
>> * It returns NULL if the block was unreadable.
>> */
>> struct buffer_head *
>> __bread(struct block_device *bdev, sector_t block, unsigned size)
>> {
>> - struct buffer_head *bh = __getblk(bdev, block, size);
>> + return __bread_gfp(bdev, block, size, __GFP_MOVABLE);
>> +}
>> +EXPORT_SYMBOL(__bread);
>> +
>> +/**
>> + * __bread_gfp() - reads a specified block and returns the bh
>> + * @bdev: the block_device to read from
>> + * @block: number of block
>> + * @size: size (in bytes) to read
>> + * @gfp: page allocation flag
>> + *
>> + * Reads a specified block, and returns buffer head that contains it.
>> + * The page cache can be allocated from non-movable area
>> + * not to prevent page migration if you set gfp to zero.
>> + * It returns NULL if the block was unreadable.
>> + */
>> +struct buffer_head *
>> +__bread_gfp(struct block_device *bdev, sector_t block,
>> + unsigned size, gfp_t gfp)
>> +{
>> + struct buffer_head *bh = __getblk_gfp(bdev, block, size, gfp);
>>
>> if (likely(bh) && !buffer_uptodate(bh))
>> bh = __bread_slow(bh);
>> return bh;
>> }
>> -EXPORT_SYMBOL(__bread);
>> +EXPORT_SYMBOL(__bread_gfp);
>>
>> /*
>> * invalidate_bh_lrus() is called rarely - but not only at unmount.
>> diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
>> index 324329c..a1d73fd 100644
>> --- a/include/linux/buffer_head.h
>> +++ b/include/linux/buffer_head.h
>> @@ -177,10 +177,14 @@ struct buffer_head *__find_get_block(struct block_device *bdev, sector_t block,
>> unsigned size);
>> struct buffer_head *__getblk(struct block_device *bdev, sector_t block,
>> unsigned size);
>> +struct buffer_head *__getblk_gfp(struct block_device *bdev, sector_t block,
>> + unsigned size, gfp_t gfp);
>> void __brelse(struct buffer_head *);
>> void __bforget(struct buffer_head *);
>> void __breadahead(struct block_device *, sector_t block, unsigned int size);
>> struct buffer_head *__bread(struct block_device *, sector_t block, unsigned size);
>> +struct buffer_head *__bread_gfp(struct block_device *,
>> + sector_t block, unsigned size, gfp_t gfp);
>> void invalidate_bh_lrus(void);
>> struct buffer_head *alloc_buffer_head(gfp_t gfp_flags);
>> void free_buffer_head(struct buffer_head * bh);
>> @@ -295,7 +299,13 @@ static inline void bforget(struct buffer_head *bh)
>> static inline struct buffer_head *
>> sb_bread(struct super_block *sb, sector_t block)
>> {
>> - return __bread(sb->s_bdev, block, sb->s_blocksize);
>> + return __bread_gfp(sb->s_bdev, block, sb->s_blocksize, __GFP_MOVABLE);
>> +}
>> +
>> +static inline struct buffer_head *
>> +sb_bread_gfp(struct super_block *sb, sector_t block, gfp_t gfp)
>> +{
>> + return __bread_gfp(sb->s_bdev, block, sb->s_blocksize, gfp);
>> }
> I think Andrew was suggesting to provide sb_bread_unmovable() and
> sb_getblk_unmovable() which would set appropriately. It is then more
> obvious what are filesystems trying to do when using those functions...

I think the common interface is important.

If sb_getblk_unmovable() is obvious for the filesystem,
I will add some codes for getblk_unmovable() which calling __getblk_gfp(),
and sb_bread_unmovable() calling __bread_gfp().
If so, sb_bread_gfp is not necessary.

It might be like followings:

diff --git a/fs/buffer.c b/fs/buffer.c
index 14f2f21..35caf77 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -1088,7 +1088,7 @@ grow_buffers(struct block_device *bdev, sector_t block, int siz
return grow_dev_page(bdev, block, index, size, sizebits, gfp);
}

-struct buffer_head *
+static struct buffer_head *
__getblk_gfp(struct block_device *bdev, sector_t block,
unsigned size, gfp_t gfp)
{
@@ -1119,7 +1119,13 @@ __getblk_gfp(struct block_device *bdev, sector_t block,
free_more_memory();
}
}
-EXPORT_SYMBOL(__getblk_gfp);
+
+struct buffer_head *getblk_unmovable(struct block_device *bdev, sector_t block,
+ unsigned size)
+{
+ return __getblk_gfp(bdev, block, size, 0);
+}
+EXPORT_SYMBOL(getblk_unmovable);

/*
* The relationship between dirty buffers and dirty pages:
diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
index a1d73fd..c5fb4fc 100644
--- a/include/linux/buffer_head.h
+++ b/include/linux/buffer_head.h
@@ -177,8 +177,8 @@ struct buffer_head *__find_get_block(struct block_device *bdev, s
unsigned size);
struct buffer_head *__getblk(struct block_device *bdev, sector_t block,
unsigned size);
-struct buffer_head *__getblk_gfp(struct block_device *bdev, sector_t block,
- unsigned size, gfp_t gfp);
+struct buffer_head *getblk_unmovable(struct block_device *bdev, sector_t block,
+ unsigned size);
void __brelse(struct buffer_head *);
void __bforget(struct buffer_head *);
void __breadahead(struct block_device *, sector_t block, unsigned int size);
@@ -303,9 +303,9 @@ sb_bread(struct super_block *sb, sector_t block)
}

static inline struct buffer_head *
-sb_bread_gfp(struct super_block *sb, sector_t block, gfp_t gfp)
+sb_bread_unmovable(struct super_block *sb, sector_t block)
{
- return __bread_gfp(sb->s_bdev, block, sb->s_blocksize, gfp);
+ return __bread_gfp(sb->s_bdev, block, sb->s_blocksize, 0);
}

static inline void

Is it better?

Thank you for your advice.