Re: [f2fs-dev] Why use page_cache_ra_unbounded?

[Daeho Jeong <daeho43@gmail.com>]

2021년 4월 8일 (목) 오전 12:05, Matthew Wilcox <willy@infradead.org>님이 작성:
>
>
> commit 5fdb322ff2c2b4ad519f490dcb7ebb96c5439af7
> Author: Daeho Jeong <daehojeong@google.com>
> Date:   Thu Dec 3 15:56:15 2020 +0900
>
>     f2fs: add F2FS_IOC_DECOMPRESS_FILE and F2FS_IOC_COMPRESS_FILE
>
> +       page_cache_ra_unbounded(&ractl, len, 0);
>
> /**
>  * page_cache_ra_unbounded - Start unchecked readahead.
>  * @ractl: Readahead control.
>  * @nr_to_read: The number of pages to read.
>  * @lookahead_size: Where to start the next readahead.
>  *
>  * This function is for filesystems to call when they want to start
>  * readahead beyond a file's stated i_size.  This is almost certainly
>  * not the function you want to call.  Use page_cache_async_readahead()
>  * or page_cache_sync_readahead() instead.
>  *
>  * Context: File is referenced by caller.  Mutexes may be held by caller.
>  * May sleep, but will not reenter filesystem to reclaim memory.
>  */
>
> Why?
>

Hi Matthew,

What I wanted here is like do_page_cache_ra(), but do_page_cache_ra()
is defined in mm/internal.h only for internal use.
So, I used it, because we already checked the i_size boundary on our own.
Actually, I wanted to detour the internal readahead mechanism using
page_cache_ra_unbounded() to generate cluster size aligned read requests.
Plus, page_cache_sync_readahead() is not good for our situation,
it might end up with cluster misaligned reads which trigger internal
duplicated cluster reads.

>
>
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