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SubjectRe: [PATCH v20 3/7 RESEND] xbitmap: add more operations
On Sat, Dec 23, 2017 at 11:59:54AM +0900, Tetsuo Handa wrote:
> Matthew Wilcox wrote:
> > + bit %= IDA_BITMAP_BITS;
> > + radix_tree_iter_init(&iter, index);
> > + slot = idr_get_free_cmn(root, &iter, GFP_NOWAIT | __GFP_NOWARN, index);
> > + if (IS_ERR(slot)) {
> > + if (slot == ERR_PTR(-ENOSPC))
> > + return 0; /* Already set */
>
> Why already set? I guess something is there, but is it guaranteed that
> there is a bitmap with the "bit" set?

Yes. For radix trees tagged with IDR_RT_MARKER, newly created slots
have the IDR_FREE tag set. We only clear the IDR_FREE tag once the
bitmap is full. So if we try to find a free slot and the tag is clear,
we know the bitmap is full.

> > + bitmap = rcu_dereference_raw(*slot);
> > + if (!bitmap) {
> > + bitmap = this_cpu_xchg(ida_bitmap, NULL);
> > + if (!bitmap)
> > + return -ENOMEM;
>
> I can't understand this. I can understand if it were
>
> BUG_ON(!bitmap);
>
> because you called xb_preload().
>
> But
>
> /*
> * Regular test 2
> * set bit 2000, 2001, 2040
> * Next 1 in [0, 2048) --> 2000
> * Next 1 in [2000, 2002) --> 2000
> * Next 1 in [2002, 2041) --> 2040
> * Next 1 in [2002, 2040) --> none
> * Next 0 in [2000, 2048) --> 2002
> * Next 0 in [2048, 2060) --> 2048
> */
> xb_preload(GFP_KERNEL);
> assert(!xb_set_bit(&xb1, 2000));
> assert(!xb_set_bit(&xb1, 2001));
> assert(!xb_set_bit(&xb1, 2040));
[...]
> xb_preload_end();
>
> you are not calling xb_preload() prior to each xb_set_bit() call.
> This means that, if each xb_set_bit() is not surrounded with
> xb_preload()/xb_preload_end(), there is possibility of hitting
> this_cpu_xchg(ida_bitmap, NULL) == NULL.

This is just a lazy test. We "know" that the bits in the range 1024-2047
will all land in the same bitmap, so there's no need to preload for each
of them.

> If bitmap == NULL at this_cpu_xchg(ida_bitmap, NULL) is allowed,
> you can use kzalloc(sizeof(*bitmap), GFP_NOWAIT | __GFP_NOWARN)
> and get rid of xb_preload()/xb_preload_end().

No, we can't. GFP_NOWAIT | __GFP_NOWARN won't try very hard to allocate
memory. There's no reason to fail the call if the user is in a context
where they can try harder to free memory.

> You are using idr_get_free_cmn(GFP_NOWAIT | __GFP_NOWARN), which
> means that the caller has to be prepared for allocation failure
> when calling xb_set_bit(). Thus, there is no need to use preload
> in order to avoid failing to allocate "bitmap".

xb_preload also preloads radix tree nodes.

> Also, please clarify why it is OK to just return here.
> I don't know what
>
> radix_tree_iter_replace(root, &iter, slot, bitmap);
>
> is doing. If you created a slot but did not assign "bitmap",
> what the caller of xb_test_bit() etc. will find? If there is an
> assumption about this slot, won't this cause a problem?

xb_test_bit will find NULL if bitmap wasn't assigned. That doesn't
harm anything.

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