Re: why the kmalloc return fail when there is free physical address but return success after dropping page caches

[Vlastimil Babka <vbabka@suse.cz>]

[+CC Dave]

On 05/18/2016 04:38 AM, baotiao wrote:
> Hello every, I meet an interesting kernel memory problem. Can anyone
> help me explain what happen under the kernel

Which kernel version is that?

> The machine's status is describe as blow:
>
> the machine has 96 physical memory. And the real use memory is about
> 64G, and the page cache use about 32G. we also use the swap area, at
> that time we have about 10G(we set the swap max size to 32G). At that
> moment, we find xfs report
>
> |Apr 29 21:54:31 w-openstack86 kernel: XFS: possible memory allocation
> deadlock in kmem_alloc (mode:0x250) |

Just once, or many times?

> after reading the source code. This message is display from this line
>
> |ptr = kmalloc(size, lflags); if (ptr || (flags &
> (KM_MAYFAIL|KM_NOSLEEP))) return ptr; if (!(++retries % 100))
> xfs_err(NULL, "possible memory allocation deadlock in %s (mode:0x%x)",
> __func__, lflags); congestion_wait(BLK_RW_ASYNC, HZ/50); |

Any indication what is the size used here?

> The error is cause by the kmalloc() function, there is not enough memory
> in the system. But there is still 32G page cache.
>
> So I run
>
> |echo 3 > /proc/sys/vm/drop_caches |
>
> to drop the page cache.
>
> Then the system is fine.

Are you saying that the error message was repeated infinitely until you 
did the drop_caches?

> But I really don't know the reason. Why after I
> run drop_caches operation the kmalloc() function will success? I think
> even we use whole physical memory, but we only use 64 real momory, the
> 32G memory are page cache, further we have enough swap space. So why the
> kernel don't flush the page cache or the swap to reserved the kmalloc
> operation.
>
>
> ----------------------------------------
> Github: https://github.com/baotiao
> Blog: http://baotiao.github.io/
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>

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