data integrity check needed

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write_cache_pages

WRITE_CACHE_PAGES(9)					    Memory Management in Linux					      WRITE_CACHE_PAGES(9)

NAME

       write_cache_pages - walk the list of dirty pages of the given address space and write all of them.

SYNOPSIS

       int write_cache_pages(struct address_space * mapping, struct writeback_control * wbc, writepage_t writepage, void * data);

ARGUMENTS

       mapping
	   address space structure to write

       wbc
	   subtract the number of written pages from *wbc->nr_to_write

       writepage
	   function called for each page

       data
	   data passed to writepage function

DESCRIPTION

       If a page is already under I/O, write_cache_pages skips it, even if it's dirty. This is desirable behaviour for memory-cleaning writeback,
       but it is INCORRECT for data-integrity system calls such as fsync.  fsync and msync need to guarantee that all the data which was dirty at
       the time the call was made get new I/O started against them. If wbc->sync_mode is WB_SYNC_ALL then we were called for data integrity and we
       must wait for existing IO to complete.

       To avoid livelocks (when other process dirties new pages), we first tag pages which should be written back with TOWRITE tag and only then
       start writing them. For data-integrity sync we have to be careful so that we do not miss some pages (e.g., because some other process has
       cleared TOWRITE tag we set). The rule we follow is that TOWRITE tag can be cleared only by the process clearing the DIRTY tag (and
       submitting the page for IO).

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Kernel Hackers Manual 3.10					     June 2014						      WRITE_CACHE_PAGES(9)

Shell Programming and Scripting