Re: [PATCH 09/12] thp: introduce khugepaged_prealloc_page and khugepaged_alloc_page

From: Xiao Guangrong
Date: Tue Sep 11 2012 - 22:36:35 EST

On 09/12/2012 10:03 AM, Hugh Dickins wrote:
> On Mon, 13 Aug 2012, Xiao Guangrong wrote:
>> They are used to abstract the difference between NUMA enabled and NUMA disabled
>> to make the code more readable
>> Signed-off-by: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxxxxxx>
>> ---
>> mm/huge_memory.c | 166 ++++++++++++++++++++++++++++++++----------------------
>> 1 files changed, 98 insertions(+), 68 deletions(-)
> Hmm, that in itself is not necessarily an improvement.
> I'm a bit sceptical about this patch,
> thp-introduce-khugepaged_prealloc_page-and-khugepaged_alloc_page.patch
> in last Thursday's mmotm 2012-09-06-16-46.
> What brought me to look at it was hitting "BUG at mm/huge_memory.c:1842!"
> running tmpfs kbuild swapping load (with memcg's memory.limit_in_bytes
> forcing out to swap), while I happened to have CONFIG_NUMA=y.
> That's the VM_BUG_ON(*hpage) on entry to khugepaged_alloc_page().

I will look into it, thanks for your point it out.

> (If I'm honest, I'll admit I have Michel's "interval trees for anon rmap"
> patches in on top, and so the line number was actually shifted to 1839:
> but I don't believe his patches were in any way involved here, and
> indeed I've not yet found a problem with them: they look very good.)
> I expect the BUG could quite easily be fixed up by making another call
> to khugepaged_prealloc_page() from somewhere to free up the hpage;
> but forgive me if I dislike using "prealloc" to free.
> I do agree with you that the several CONFIG_NUMA ifdefs dotted around
> mm/huge_memory.c are regrettable, but I'm not at all sure that you're
> improving the situation with this patch, which gives misleading names
> to functions and moves the mmap_sem upping out of line.
> I think you need to revisit it: maybe not go so far (leaving a few
> CONFIG_NUMAs behind, if they're not too bad), or maybe go further
> (add a separate function for freeing in the NUMA case, instead of
> using "prealloc"). I don't know what's best: have a play and see.

Sorry for that, i will find a better way to do this.

> That's what I was intending to write yesterday. But overnight I
> was running with this 9/12 backed out (I think 10,11,12 should be
> independent), and found "BUG at mm/huge_memory.c:1835!" this morning.
> That's the VM_BUG_ON(*hpage) below #else in collapse_huge_page()
> when 9/12 is reverted.
> So maybe 9/12 is just obscuring what was already a BUG, either earlier
> in your series or elsewhere in mmotm (I've never seen it on 3.6-rc or
> earlier releases, nor without CONFIG_NUMA). I've not spent any time
> looking for it, maybe it's obvious - can you spot and fix it?

Sure, will fix it as soon as possible. Thanks!

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