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    SubjectQuestions: bforget & forgotten pages

    For nefarious reasons of my own, I'm interested in keeping buffer usage
    constant, in a system with only the rd block device. I've started out by
    populating the ram disk with an un-holey filesystem image, and from there
    it does a good job of not using more or less memory -- with one exception.

    I finally figured out what was causing a bizarre "leak": the bforget
    function which is invoked by ext2/truncate.c -- and _only_ by that code,
    at least in 2.0.33. In the process of truncating a file, bforget is
    invoked on the discarded buffers, which removes the protected bits (thus
    turning the pages loose from the ram disk), and resets the device numbers
    to zero without moving the page to the free list. The former is just
    annoying, but the latter seems to have the fairly bizarre effect of
    leaving "zombie" clean buffers around that, as far as I can tell, will not
    be reclaimed or reused by anything short of try_to_free_buffer, or the
    inner machinations of refill_freelist.

    So, two questions: can I replace bforget in the ext2 code with something
    else, brelse perhaps? (It's not obvious whether their semantics are
    identical.) Secondly, am I mistaken about forgotten pages, and are they
    reclaimable via some process I've missed?

    (I've not tried to demonstrate this on a stock Linux system, but the
    principle is simple enough: create a ramdisk, create an ext2fs on that
    ramdisk, create a file on that fs, then truncate that file. Some of the
    ramdisk buffers will transmute into clean buffers with dev=NULLDEV. More
    interestingly, if you repeatedly create and truncate a file, you'll
    continually spawn more forgotten buffers. I'm not positive that this will
    happen without the ram disk. I'm fairly sure that all of this isn't just
    the symptom of some bug of mine, but I can't swear to that either,
    unfortunately.)

    Thanks for any assistance,
    Ken
    --
    Kenneth Albanowski (kjahds@kjahds.com, CIS: 70705,126)



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