Messages in this thread | | | Date | Mon, 28 May 2007 14:54:16 +1000 | From | Nick Piggin <> | Subject | Re: [PATCH 1/3] Preserve the dirty bit in init_page_buffers |
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Eric W. Biederman wrote: > Nick Piggin <nickpiggin@yahoo.com.au> writes: > > >>Eric W. Biederman wrote: >> >>>The problem: When we are trying to free buffers try_to_free_buffers >>>will look at ramdisk pages with clean buffer heads and remove the >>>dirty bit from the page. Resulting in ramdisk pages with data that >>>get removed from the page cache. Ouch! >>> >>>Buffer heads appear on ramdisk pages when a filesystem calls getblk, >>>which through a series of function calls eventually calls >>>init_page_buffers. >>> >>>So to fix the mismatch between buffer head state and page state this >>>patch modifies init_page_buffers to transfer the dirty bit from the >>>page to the buffer heads like we currently do for the uptodate bit. >> >>Ouch indeed! >> >>But can we ever have a dirty page at init_page_buffers-time? > > > Definitely, and it was a royal pain to trace the bug that this > caused. An initial ramdisk having pieces disappear after mkfs > is called can look like the entire machine is dying. > > When we initialize the ramdisk by writing to /dev/ram0 usually in > init/do_mounts_rd.c we don't allocate buffer heads but we do set > the dirty bit, and the page is in the page cache. So when we > later call getblk it reuses the same page and then calls > init_page_buffers.
Hmm, so this would be a problem for block_dev.c as well, then? Because it would be possible to have a dirty block dev page have its buffers reclaimed and then reinitialised via init_page_buffers, AFAIKS.
>>I would have thought we can fix this simply by removing the >>broken ramdisk_set_page_dirty (as far as the comment goes, we >>set CAP_NO_ACCT_DIRTY anyway, so the normal set_page_dirty >>should handle everything properly, no?). > > > No. I don't know where accounting comes into play. I didn't > trace that path. But if we have a non-dirty ramdisk page with > buffers (basically a hole in the middle or at the end of the ramdisk). > We need to set the buffer dirty bits when we write to it.
Accounting is done in set_page_dirty.
> > So I don't see how it would make sense to reuse the generic > set_page_dirty, and handling all of the logic in set_page_dirty > to dirty the buffer heads seemed to have made the most sense.
That's what the generic set_page_dirty does. What I want to know is why *doesn't* it make sense to reuse the generic set_page_dirty? Unless there is a good reason, then reusing is better than writing your own.
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