Nick Piggin <[email protected]> writes:
> On Saturday 20 October 2007 08:51, Eric W. Biederman wrote:
>> Currently the ramdisk tries to keep the block device page cache pages
>> from being marked clean and dropped from memory. That fails for
>> filesystems that use the buffer cache because the buffer cache is not
>> an ordinary buffer cache user and depends on the generic block device
>> address space operations being used.
>>
>> To fix all of those associated problems this patch allocates a private
>> inode to store the ramdisk pages in.
>>
>> The result is slightly more memory used for metadata, an extra copying
>> when reading or writing directly to the block device, and changing the
>> software block size does not loose the contents of the ramdisk. Most
>> of all this ensures we don't loose data during normal use of the
>> ramdisk.
>>
>> I deliberately avoid the cleanup that is now possible because this
>> patch is intended to be a bug fix.
>
> This just breaks coherency again like the last patch. That's a
> really bad idea especially for stable (even if nothing actually
> was to break, we'd likely never know about it anyway).
Not a chance. The only way we make it to that inode is through block
device I/O so it lives at exactly the same level in the hierarchy as
a real block device. My patch is the considered rewrite boiled down
to it's essentials and made a trivial patch.
It fundamentally fixes the problem, and doesn't attempt to reconcile
the incompatible expectations of the ramdisk code and the buffer cache.
> Christian's patch should go upstream and into stable. For 2.6.25-6,
> my rewrite should just replace what's there. Using address spaces
> to hold the ramdisk pages just confuses the issue even if they
> *aren't* actually wired up to the vfs at all. Saving 20 lines is
> not a good reason to use them.
Well is more like saving 100 lines. Not having to reexamine complicated
infrastructure code and doing things the same way ramfs is. I think
that combination is a good reason. Especially since I can do with a
16 line patch as I just demonstrated. It is a solid and simple
incremental change.
Eric
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