Re: [PATCH] mm: readahead: remove redundant ra_pages in file_ra_state

[Ni zhan Chen <nizhan.chen@gmail.com>]

On 10/23/2012 08:46 PM, Ying Zhu wrote:
> Hi,
>    Recently we ran into the bug that an opened file's ra_pages does not
> synchronize with it's backing device's when the latter is changed
> with blockdev --setra, the application needs to reopen the file
> to know the change, which is inappropriate under our circumstances.

Could you tell me in which function do this synchronize stuff?

> This bug is also mentioned in scst (generic SCSI target subsystem for Linux)'s
> README file.
>    This patch tries to unify the ra_pages in struct file_ra_state
> and struct backing_dev_info. Basically current readahead algorithm
> will ramp file_ra_state.ra_pages up to bdi.ra_pages once it detects the

You mean ondemand readahead algorithm will do this? I don't think so. 
file_ra_state_init only called in btrfs path, correct?

> read mode is sequential. Then all files sharing the same backing device
> have the same max value bdi.ra_pages set in file_ra_state.

why remove file_ra_state? If one file is read sequential and another 
file is read ramdom, how can use the global bdi.ra_pages to indicate the 
max readahead window of each file?

>    Applying this means the flags POSIX_FADV_NORMAL and POSIX_FADV_SEQUENTIAL
> in fadivse will only set file reading mode without signifying the
> max readahead size of the file. The current apporach adds no additional
> overhead in read IO path, IMHO is the simplest solution.
> Any comments are welcome, thanks in advance.

Could you show me how you test this patch?

>
> Thanks,
> 	Ying Zhu
>
> Signed-off-by: Ying Zhu <casualfisher@gmail.com>
> ---
>   include/linux/fs.h |    1 -
>   mm/fadvise.c       |    2 --
>   mm/filemap.c       |   17 +++++++++++------
>   mm/readahead.c     |    8 ++++----
>   4 files changed, 15 insertions(+), 13 deletions(-)
>
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 17fd887..36303a5 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -991,7 +991,6 @@ struct file_ra_state {
>   	unsigned int async_size;	/* do asynchronous readahead when
>   					   there are only # of pages ahead */
>   
> -	unsigned int ra_pages;		/* Maximum readahead window */
>   	unsigned int mmap_miss;		/* Cache miss stat for mmap accesses */
>   	loff_t prev_pos;		/* Cache last read() position */
>   };
> diff --git a/mm/fadvise.c b/mm/fadvise.c
> index 469491e..75e2378 100644
> --- a/mm/fadvise.c
> +++ b/mm/fadvise.c
> @@ -76,7 +76,6 @@ SYSCALL_DEFINE(fadvise64_64)(int fd, loff_t offset, loff_t len, int advice)
>   
>   	switch (advice) {
>   	case POSIX_FADV_NORMAL:
> -		file->f_ra.ra_pages = bdi->ra_pages;
>   		spin_lock(&file->f_lock);
>   		file->f_mode &= ~FMODE_RANDOM;
>   		spin_unlock(&file->f_lock);
> @@ -87,7 +86,6 @@ SYSCALL_DEFINE(fadvise64_64)(int fd, loff_t offset, loff_t len, int advice)
>   		spin_unlock(&file->f_lock);
>   		break;
>   	case POSIX_FADV_SEQUENTIAL:
> -		file->f_ra.ra_pages = bdi->ra_pages * 2;
>   		spin_lock(&file->f_lock);
>   		file->f_mode &= ~FMODE_RANDOM;
>   		spin_unlock(&file->f_lock);
> diff --git a/mm/filemap.c b/mm/filemap.c
> index a4a5260..e7e4409 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1058,11 +1058,15 @@ EXPORT_SYMBOL(grab_cache_page_nowait);
>    * readahead(R+4...B+3) => bang => read(R+4) => read(R+5) => ......
>    *
>    * It is going insane. Fix it by quickly scaling down the readahead size.
> + * It's hard to estimate how the bad sectors lay out, so to be conservative,
> + * set the read mode in random.
>    */
>   static void shrink_readahead_size_eio(struct file *filp,
>   					struct file_ra_state *ra)
>   {
> -	ra->ra_pages /= 4;
> +	spin_lock(&filp->f_lock);
> +	filp->f_mode |= FMODE_RANDOM;
> +	spin_unlock(&filp->f_lock);
>   }
>   
>   /**
> @@ -1527,12 +1531,12 @@ static void do_sync_mmap_readahead(struct vm_area_struct *vma,
>   	/* If we don't want any read-ahead, don't bother */
>   	if (VM_RandomReadHint(vma))
>   		return;
> -	if (!ra->ra_pages)
> +	if (!mapping->backing_dev_info->ra_pages)
>   		return;
>   
>   	if (VM_SequentialReadHint(vma)) {
> -		page_cache_sync_readahead(mapping, ra, file, offset,
> -					  ra->ra_pages);
> +		page_cache_sync_readahead(mapping, ra, file, offset,
> +					mapping->backing_dev_info->ra_pages);
>   		return;
>   	}
>   
> @@ -1550,7 +1554,7 @@ static void do_sync_mmap_readahead(struct vm_area_struct *vma,
>   	/*
>   	 * mmap read-around
>   	 */
> -	ra_pages = max_sane_readahead(ra->ra_pages);
> +	ra_pages = max_sane_readahead(mapping->backing_dev_info->ra_pages);
>   	ra->start = max_t(long, 0, offset - ra_pages / 2);
>   	ra->size = ra_pages;
>   	ra->async_size = ra_pages / 4;
> @@ -1576,7 +1580,8 @@ static void do_async_mmap_readahead(struct vm_area_struct *vma,
>   		ra->mmap_miss--;
>   	if (PageReadahead(page))
>   		page_cache_async_readahead(mapping, ra, file,
> -					   page, offset, ra->ra_pages);
> +					   page, offset,
> +					   mapping->backing_dev_info->ra_pages);
>   }
>   
>   /**
> diff --git a/mm/readahead.c b/mm/readahead.c
> index ea8f8fa..6ea5999 100644
> --- a/mm/readahead.c
> +++ b/mm/readahead.c
> @@ -27,7 +27,6 @@
>   void
>   file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping)
>   {
> -	ra->ra_pages = mapping->backing_dev_info->ra_pages;
>   	ra->prev_pos = -1;
>   }
>   EXPORT_SYMBOL_GPL(file_ra_state_init);
> @@ -400,7 +399,8 @@ ondemand_readahead(struct address_space *mapping,
>   		   bool hit_readahead_marker, pgoff_t offset,
>   		   unsigned long req_size)
>   {
> -	unsigned long max = max_sane_readahead(ra->ra_pages);
> +	struct backing_dev_info *bdi = mapping->backing_dev_info;
> +	unsigned long max = max_sane_readahead(bdi->ra_pages);
>   
>   	/*
>   	 * start of file
> @@ -507,7 +507,7 @@ void page_cache_sync_readahead(struct address_space *mapping,
>   			       pgoff_t offset, unsigned long req_size)
>   {
>   	/* no read-ahead */
> -	if (!ra->ra_pages)
> +	if (!mapping->backing_dev_info->ra_pages)
>   		return;
>   
>   	/* be dumb */
> @@ -543,7 +543,7 @@ page_cache_async_readahead(struct address_space *mapping,
>   			   unsigned long req_size)
>   {
>   	/* no read-ahead */
> -	if (!ra->ra_pages)
> +	if (!mapping->backing_dev_info->ra_pages)
>   		return;
>   
>   	/*

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