Re: [PATCH v2 3/4] powerpc/mm: fix a warning when a cache is common to PGD and hugepages

[Aneesh Kumar K.V]

On 08/14/2018 08:24 PM, Christophe Leroy wrote:
> While implementing TLB miss HW assistance on the 8xx, the following
> warning was encountered:
>
> [  423.732965] WARNING: CPU: 0 PID: 345 at mm/slub.c:2412 ___slab_alloc.constprop.30+0x26c/0x46c
> [  423.733033] CPU: 0 PID: 345 Comm: mmap Not tainted 4.18.0-rc8-00664-g2dfff9121c55 #671
> [  423.733075] NIP:  c0108f90 LR: c0109ad0 CTR: 00000004
> [  423.733121] REGS: c455bba0 TRAP: 0700   Not tainted  (4.18.0-rc8-00664-g2dfff9121c55)
> [  423.733147] MSR:  00021032 <ME,IR,DR,RI>  CR: 24224848  XER: 20000000
> [  423.733319]
> [  423.733319] GPR00: c0109ad0 c455bc50 c4521910 c60053c0 007080c0 c0011b34 c7fa41e0 c455be30
> [  423.733319] GPR08: 00000001 c00103a0 c7fa41e0 c49afcc4 24282842 10018840 c079b37c 00000040
> [  423.733319] GPR16: 73f00000 00210d00 00000000 00000001 c455a000 00000100 00000200 c455a000
> [  423.733319] GPR24: c60053c0 c0011b34 007080c0 c455a000 c455a000 c7fa41e0 00000000 00009032
> [  423.734190] NIP [c0108f90] ___slab_alloc.constprop.30+0x26c/0x46c
> [  423.734257] LR [c0109ad0] kmem_cache_alloc+0x210/0x23c
> [  423.734283] Call Trace:
> [  423.734326] [c455bc50] [00000100] 0x100 (unreliable)
> [  423.734430] [c455bcc0] [c0109ad0] kmem_cache_alloc+0x210/0x23c
> [  423.734543] [c455bcf0] [c0011b34] huge_pte_alloc+0xc0/0x1dc
> [  423.734633] [c455bd20] [c01044dc] hugetlb_fault+0x408/0x48c
> [  423.734720] [c455bdb0] [c0104b20] follow_hugetlb_page+0x14c/0x44c
> [  423.734826] [c455be10] [c00e8e54] __get_user_pages+0x1c4/0x3dc
> [  423.734919] [c455be80] [c00e9924] __mm_populate+0xac/0x140
> [  423.735020] [c455bec0] [c00db14c] vm_mmap_pgoff+0xb4/0xb8
> [  423.735127] [c455bf00] [c00f27c0] ksys_mmap_pgoff+0xcc/0x1fc
> [  423.735222] [c455bf40] [c000e0f8] ret_from_syscall+0x0/0x38
> [  423.735271] Instruction dump:
> [  423.735321] 7cbf482e 38fd0008 7fa6eb78 7fc4f378 4bfff5dd 7fe3fb78 4bfffe24 81370010
> [  423.735536] 71280004 41a2ff88 4840c571 4bffff80 <0fe00000> 4bfffeb8 81340010 712a0004
> [  423.735757] ---[ end trace e9b222919a470790 ]---
>
> This warning occurs when calling kmem_cache_zalloc() on a
> cache having a constructor.
>
> In this case it happens because PGD cache and 512k hugepte cache are
> the same size (4k). While a cache with constructor is created for
> the PGD, hugepages create cache without constructor and uses
> kmem_cache_zalloc(). As both expect a cache with the same size,
> the hugepages reuse the cache created for PGD, hence the conflict.
>
> In order to avoid this conflict, this patch:
> - modifies pgtable_cache_add() so that a zeroising constructor is
> added for any cache size.
> - replaces calls to kmem_cache_zalloc() by kmem_cache_alloc()

Can't we just do kmem_cache_alloc with gfp flags __GFP_ZERO? and remove 
the constructor completely?


-aneesh