Re: how do file-mapped (mmapped) pages become dirty?

[Chris Friesen]

Richard B. Johnson wrote:
> On Tue, 28 Oct 2003, Chris Friesen wrote:

> > Note however that you need an msync() -- fsync() and fdatasync() do not
> > catch changes to mmapped pages.

> Sure they do. fsync() will sync the whole file, regardless of
> whether or not it's been mapped. msync()  allows you to sync
> only a  specific portion and control how that portion is
> handled with some flags.

According to Rik van Riel and HPA, fsync() does not check page tables 
for dirty bits.  This is confirmed by my testing, in which msync() takes 
significantly longer than fsync() for the same file (in a ramdisk). 
msync() on the 3 pages dirtied took 39 usec, while fsync() on the whole 
200KB file took 12 usec.

Also, some googling leads to to believe that if the file is mmapped and 
you close() it, it won't get synced.

Chris

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