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Message-ID: <8bfed1e2-ec44-473b-b006-8cb2505220d4@lucifer.local>
Date: Tue, 5 Aug 2025 15:43:26 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Usama Arif <usamaarif642@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, david@...hat.com,
linux-mm@...ck.org, linux-fsdevel@...r.kernel.org, corbet@....net,
rppt@...nel.org, surenb@...gle.com, mhocko@...e.com,
hannes@...xchg.org, baohua@...nel.org, shakeel.butt@...ux.dev,
riel@...riel.com, ziy@...dia.com, laoar.shao@...il.com,
dev.jain@....com, baolin.wang@...ux.alibaba.com, npache@...hat.com,
Liam.Howlett@...cle.com, ryan.roberts@....com, vbabka@...e.cz,
jannh@...gle.com, Arnd Bergmann <arnd@...db.de>, sj@...nel.org,
linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
kernel-team@...a.com
Subject: Re: [PATCH v3 3/6] mm/huge_memory: respect MADV_COLLAPSE with
PR_THP_DISABLE_EXCEPT_ADVISED
On Mon, Aug 04, 2025 at 04:40:46PM +0100, Usama Arif wrote:
> From: David Hildenbrand <david@...hat.com>
>
> Let's allow for making MADV_COLLAPSE succeed on areas that neither have
> VM_HUGEPAGE nor VM_NOHUGEPAGE when we have THP disabled
> unless explicitly advised (PR_THP_DISABLE_EXCEPT_ADVISED).
>
> MADV_COLLAPSE is a clear advice that we want to collapse.
>
> Note that we still respect the VM_NOHUGEPAGE flag, just like
> MADV_COLLAPSE always does. So consequently, MADV_COLLAPSE is now only
> refused on VM_NOHUGEPAGE with PR_THP_DISABLE_EXCEPT_ADVISED,
> including for shmem.
Hm feels like 'including for shmem' is a bit brief here :)
But fine probably ok.
>
> Co-developed-by: Usama Arif <usamaarif642@...il.com>
> Signed-off-by: Usama Arif <usamaarif642@...il.com>
> Signed-off-by: David Hildenbrand <david@...hat.com>
LGTM, so:
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> ---
> include/linux/huge_mm.h | 8 +++++++-
> include/uapi/linux/prctl.h | 2 +-
> mm/huge_memory.c | 5 +++--
> mm/memory.c | 6 ++++--
> mm/shmem.c | 2 +-
> 5 files changed, 16 insertions(+), 7 deletions(-)
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index bd4f9e6327e0..1fd06ecbde72 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -329,7 +329,7 @@ struct thpsize {
> * through madvise or prctl.
> */
> static inline bool vma_thp_disabled(struct vm_area_struct *vma,
> - vm_flags_t vm_flags)
> + vm_flags_t vm_flags, bool forced_collapse)
> {
> /* Are THPs disabled for this VMA? */
> if (vm_flags & VM_NOHUGEPAGE)
> @@ -343,6 +343,12 @@ static inline bool vma_thp_disabled(struct vm_area_struct *vma,
> */
> if (vm_flags & VM_HUGEPAGE)
> return false;
> + /*
> + * Forcing a collapse (e.g., madv_collapse), is a clear advice to
> + * use THPs.
> + */
> + if (forced_collapse)
> + return false;
> return test_bit(MMF_DISABLE_THP_EXCEPT_ADVISED, &vma->vm_mm->flags);
> }
>
> diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
> index 9c1d6e49b8a9..cdda963a039a 100644
> --- a/include/uapi/linux/prctl.h
> +++ b/include/uapi/linux/prctl.h
> @@ -185,7 +185,7 @@ struct prctl_mm_map {
> #define PR_SET_THP_DISABLE 41
> /*
> * Don't disable THPs when explicitly advised (e.g., MADV_HUGEPAGE /
> - * VM_HUGEPAGE).
> + * VM_HUGEPAGE, MADV_COLLAPSE).
> */
> # define PR_THP_DISABLE_EXCEPT_ADVISED (1 << 1)
> #define PR_GET_THP_DISABLE 42
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 85252b468f80..ef5ccb0ec5d5 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -104,7 +104,8 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
> {
> const bool smaps = type == TVA_SMAPS;
> const bool in_pf = type == TVA_PAGEFAULT;
> - const bool enforce_sysfs = type != TVA_FORCED_COLLAPSE;
> + const bool forced_collapse = type == TVA_FORCED_COLLAPSE;
> + const bool enforce_sysfs = !forced_collapse;
I guess as discussed we'll return to this.
> unsigned long supported_orders;
>
> /* Check the intersection of requested and supported orders. */
> @@ -122,7 +123,7 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
> if (!vma->vm_mm) /* vdso */
> return 0;
>
> - if (thp_disabled_by_hw() || vma_thp_disabled(vma, vm_flags))
> + if (thp_disabled_by_hw() || vma_thp_disabled(vma, vm_flags, forced_collapse))
> return 0;
>
> /* khugepaged doesn't collapse DAX vma, but page fault is fine. */
> diff --git a/mm/memory.c b/mm/memory.c
> index be761753f240..bd04212d6f79 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5186,9 +5186,11 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, struct folio *folio, struct page *pa
> * It is too late to allocate a small folio, we already have a large
> * folio in the pagecache: especially s390 KVM cannot tolerate any
> * PMD mappings, but PTE-mapped THP are fine. So let's simply refuse any
> - * PMD mappings if THPs are disabled.
> + * PMD mappings if THPs are disabled. As we already have a THP ...
> + * behave as if we are forcing a collapse.
> */
> - if (thp_disabled_by_hw() || vma_thp_disabled(vma, vma->vm_flags))
> + if (thp_disabled_by_hw() || vma_thp_disabled(vma, vma->vm_flags,
> + /* forced_collapse=*/ true))
> return ret;
>
> if (!thp_vma_suitable_order(vma, haddr, PMD_ORDER))
> diff --git a/mm/shmem.c b/mm/shmem.c
> index e6cdfda08aed..30609197a266 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1816,7 +1816,7 @@ unsigned long shmem_allowable_huge_orders(struct inode *inode,
> vm_flags_t vm_flags = vma ? vma->vm_flags : 0;
> unsigned int global_orders;
>
> - if (thp_disabled_by_hw() || (vma && vma_thp_disabled(vma, vm_flags)))
> + if (thp_disabled_by_hw() || (vma && vma_thp_disabled(vma, vm_flags, shmem_huge_force)))
> return 0;
>
> global_orders = shmem_huge_global_enabled(inode, index, write_end,
> --
> 2.47.3
>
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