linux-kernel - Re: [PATCH v11 00/21] Add support for NV-DIMMs to ext4

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Message-ID: <9487.1412183841@turing-police.cc.vt.edu>
Date:	Wed, 01 Oct 2014 13:17:21 -0400
From:	Valdis.Kletnieks@...edu
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	Andreas Dilger <adilger@...ger.ca>,
	Matthew Wilcox <willy@...ux.intel.com>,
	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v11 00/21] Add support for NV-DIMMs to ext4

On Wed, 01 Oct 2014 11:45:47 -0400, Jeff Moyer said:

> This sounds an awful lot like posix_fadvise' POSIX_FADV_NOREUSE flag.

Gaah. Premature click.  man posix_fadvise says this:

       In kernels before 2.6.18, POSIX_FADV_NOREUSE had the same semantics  as
       POSIX_FADV_WILLNEED.   This  was  probably  a bug; since kernel 2.6.18,
       this flag is a no-op.

and mm/fadvise.c says this:
        switch (advice) {
        case POSIX_FADV_NORMAL:
                f.file->f_ra.ra_pages = bdi->ra_pages;
                spin_lock(&f.file->f_lock);
                f.file->f_mode &= ~FMODE_RANDOM;
                spin_unlock(&f.file->f_lock);
...
	                 */
                force_page_cache_readahead(mapping, f.file, start_index,
                                           nrpages);
                break;
        case POSIX_FADV_NOREUSE:
                break;
        case POSIX_FADV_DONTNEED:
                if (!bdi_write_congested(mapping->backing_dev_info))
                        __filemap_fdatawrite_range(mapping, offset, endbyte,
                                                   WB_SYNC_NONE);

                /* First and last FULL page! */

So... not much interface there, actually.  One wonders if removing the 'break;'
and allowing a fall-through would actually be an improvement....

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