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Received: from 30.42.98.36(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0X.phNqC_1774664969 cluster:ay36) by smtp.aliyun-inc.com; Sat, 28 Mar 2026 10:29:30 +0800 Message-ID: Date: Sat, 28 Mar 2026 10:29:27 +0800 Precedence: bulk X-Mailing-List: linux-btrfs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1 02/10] mm/khugepaged: remove READ_ONLY_THP_FOR_FS check To: Zi Yan , Lance Yang Cc: ljs@kernel.org, willy@infradead.org, songliubraving@fb.com, clm@fb.com, dsterba@suse.com, viro@zeniv.linux.org.uk, brauner@kernel.org, jack@suse.cz, akpm@linux-foundation.org, david@kernel.org, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, shuah@kernel.org, linux-btrfs@vger.kernel.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org References: <6FCC0430-C98D-4D7C-8C53-F7722F1BDC4A@nvidia.com> <20260327162252.57553-1-lance.yang@linux.dev> <79053B0F-8A40-41D0-8539-0CC30C903B48@nvidia.com> From: Baolin Wang In-Reply-To: <79053B0F-8A40-41D0-8539-0CC30C903B48@nvidia.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 3/28/26 12:30 AM, Zi Yan wrote: > On 27 Mar 2026, at 12:22, Lance Yang wrote: > >> On Fri, Mar 27, 2026 at 11:00:26AM -0400, Zi Yan wrote: >>> On 27 Mar 2026, at 10:31, Lorenzo Stoakes (Oracle) wrote: >>> >>>> On Fri, Mar 27, 2026 at 10:26:53PM +0800, Baolin Wang wrote: >>>>> >>>>> >>>>> On 3/27/26 10:12 PM, Lorenzo Stoakes (Oracle) wrote: >>>>>> On Fri, Mar 27, 2026 at 09:45:03PM +0800, Baolin Wang wrote: >>>>>>> >>>>>>> >>>>>>> On 3/27/26 8:02 PM, Lorenzo Stoakes (Oracle) wrote: >>>>>>>> On Fri, Mar 27, 2026 at 05:44:49PM +0800, Baolin Wang wrote: >>>>>>>>> >>>>>>>>> >>>>>>>>> On 3/27/26 9:42 AM, Zi Yan wrote: >>>>>>>>>> collapse_file() requires FSes supporting large folio with at least >>>>>>>>>> PMD_ORDER, so replace the READ_ONLY_THP_FOR_FS check with that. shmem with >>>>>>>>>> huge option turned on also sets large folio order on mapping, so the check >>>>>>>>>> also applies to shmem. >>>>>>>>>> >>>>>>>>>> While at it, replace VM_BUG_ON with returning failure values. >>>>>>>>>> >>>>>>>>>> Signed-off-by: Zi Yan >>>>>>>>>> --- >>>>>>>>>> mm/khugepaged.c | 7 +++++-- >>>>>>>>>> 1 file changed, 5 insertions(+), 2 deletions(-) >>>>>>>>>> >>>>>>>>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >>>>>>>>>> index d06d84219e1b..45b12ffb1550 100644 >>>>>>>>>> --- a/mm/khugepaged.c >>>>>>>>>> +++ b/mm/khugepaged.c >>>>>>>>>> @@ -1899,8 +1899,11 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr, >>>>>>>>>> int nr_none = 0; >>>>>>>>>> bool is_shmem = shmem_file(file); >>>>>>>>>> - VM_BUG_ON(!IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && !is_shmem); >>>>>>>>>> - VM_BUG_ON(start & (HPAGE_PMD_NR - 1)); >>>>>>>>>> + /* "huge" shmem sets mapping folio order and passes the check below */ >>>>>>>>>> + if (mapping_max_folio_order(mapping) < PMD_ORDER) >>>>>>>>>> + return SCAN_FAIL; >>>>>>>>> >>>>>>>>> This is not true for anonymous shmem, since its large order allocation logic >>>>>>>>> is similar to anonymous memory. That means it will not call >>>>>>>>> mapping_set_large_folios() for anonymous shmem. >>>>>>>>> >>>>>>>>> So I think the check should be: >>>>>>>>> >>>>>>>>> if (!is_shmem && mapping_max_folio_order(mapping) < PMD_ORDER) >>>>>>>>> return SCAN_FAIL; >>>>>>>> >>>>>>>> Hmm but in shmem_init() we have: >>>>>>>> >>>>>>>> #ifdef CONFIG_TRANSPARENT_HUGEPAGE >>>>>>>> if (has_transparent_hugepage() && shmem_huge > SHMEM_HUGE_DENY) >>>>>>>> SHMEM_SB(shm_mnt->mnt_sb)->huge = shmem_huge; >>>>>>>> else >>>>>>>> shmem_huge = SHMEM_HUGE_NEVER; /* just in case it was patched */ >>>>>>>> >>>>>>>> /* >>>>>>>> * Default to setting PMD-sized THP to inherit the global setting and >>>>>>>> * disable all other multi-size THPs. >>>>>>>> */ >>>>>>>> if (!shmem_orders_configured) >>>>>>>> huge_shmem_orders_inherit = BIT(HPAGE_PMD_ORDER); >>>>>>>> #endif >>>>>>>> >>>>>>>> And shm_mnt->mnt_sb is the superblock used for anon shmem. Also >>>>>>>> shmem_enabled_store() updates that if necessary. >>>>>>>> >>>>>>>> So we're still fine right? >>>>>>>> >>>>>>>> __shmem_file_setup() (used for anon shmem) calls shmem_get_inode() -> >>>>>>>> __shmem_get_inode() which has: >>>>>>>> >>>>>>>> if (sbinfo->huge) >>>>>>>> mapping_set_large_folios(inode->i_mapping); >>>>>>>> >>>>>>>> Shared for both anon shmem and tmpfs-style shmem. >>>>>>>> >>>>>>>> So I think it's fine as-is. >>>>>>> >>>>>>> I'm afraid not. Sorry, I should have been clearer. >>>>>>> >>>>>>> First, anonymous shmem large order allocation is dynamically controlled via >>>>>>> the global interface (/sys/kernel/mm/transparent_hugepage/shmem_enabled) and >>>>>>> the mTHP interfaces >>>>>>> (/sys/kernel/mm/transparent_hugepage/hugepages-*kB/shmem_enabled). >>>>>>> >>>>>>> This means that during anonymous shmem initialization, these interfaces >>>>>>> might be set to 'never'. so it will not call mapping_set_large_folios() >>>>>>> because sbinfo->huge is 'SHMEM_HUGE_NEVER'. >>>>>>> >>>>>>> Even if shmem large order allocation is subsequently enabled via the >>>>>>> interfaces, __shmem_file_setup -> mapping_set_large_folios() is not called >>>>>>> again. >>>>>> >>>>>> I see your point, oh this is all a bit of a mess... >>>>>> >>>>>> It feels like entirely the wrong abstraction anyway, since at best you're >>>>>> getting a global 'is enabled'. >>>>>> >>>>>> I guess what happened before was we'd never call into this with ! r/o thp for fs >>>>>> && ! is_shmem. >>>>> >>>>> Right. >>>>> >>>>>> But now we are allowing it, but should STILL be gating on !is_shmem so yeah your >>>>>> suggestion is correct I think actualyl. >>>>>> >>>>>> I do hate: >>>>>> >>>>>> if (!is_shmem && mapping_max_folio_order(mapping) < PMD_ORDER) >>>>>> >>>>>> As a bit of code though. It's horrible. >>>>> >>>>> Indeed. >>>>> >>>>>> Let's abstract that... >>>>>> >>>>>> It'd be nice if we could find a way to clean things up in the lead up to changes >>>>>> in series like this instead of sticking with the mess, but I guess since it >>>>>> mostly removes stuff that's ok for now. >>>>> >>>>> I think this check can be removed from this patch. >>>>> >>>>> During the khugepaged's scan, it will call thp_vma_allowable_order() to >>>>> check if the VMA is allowed to collapse into a PMD. >>>>> >>>>> Specifically, within the call chain thp_vma_allowable_order() -> >>>>> __thp_vma_allowable_orders(), shmem is checked via >>>>> shmem_allowable_huge_orders(), while other FSes are checked via >>>>> file_thp_enabled(). >>> >>> But for madvise(MADV_COLLAPSE) case, IIRC, it ignores shmem huge config >>> and can perform collapse anyway. This means without !is_shmem the check >>> will break madvise(MADV_COLLAPSE). Let me know if I get it wrong, since >> >> Right. That will break MADV_COLLAPSE, IIUC. >> >> For MADV_COLLAPSE on anonymous shmem, eligibility is determined by the >> TVA_FORCED_COLLAPSE path via shmem_allowable_huge_orders(), not by >> whether the inode mapping got mapping_set_large_folios() at creation >> time. >> >> Using mmap(MAP_SHARED | MAP_ANONYMOUS): >> - create time: shmem_enabled=never, hugepages-2048kB/shmem_enabled=never >> - collapse time: shmem_enabled=never, hugepages-2048kB/shmem_enabled=always >> >> With the !is_shmem guard, collapse succeeds. Without it, the same setup >> fails with -EINVAL. Right. So my suggestion is that the check should be: if (!is_shmem && mapping_max_folio_order(mapping) < PMD_ORDER) or just keep a single VM_WARN_ONCE() here, becuase I hope the thp_vma_allowable_order() will filter out those FSes that do not support large folios. >>> I was in that TVA_FORCED_COLLAPSE email thread but does not remember >>> everything there. >>> >>> >>>> >>>> It sucks not to have an assert. Maybe in that case make it a >>>> VM_WARN_ON_ONCE(). >>> >>> Will do that as I replied to David already. >>> >>>> >>>> I hate that you're left tracing things back like that... >>>> >>>>> >>>>> For those other filesystems, Patch 5 has already added the following check, >>>>> which I think is sufficient to filter out those FSes that do not support >>>>> large folios: >>>>> >>>>> if (mapping_max_folio_order(inode->i_mapping) < PMD_ORDER) >>>>> return false; >>>> >>>> 2 < 5, we won't tolerate bisection hazards. >>>> >>>>> >>>>> >>>>>>> Anonymous shmem behaves similarly to anonymous pages: it is controlled by >>>>>>> the 'shmem_enabled' interfaces and uses shmem_allowable_huge_orders() to >>>>>>> check for allowed large orders, rather than relying on >>>>>>> mapping_max_folio_order(). >>>>>>> >>>>>>> The mapping_max_folio_order() is intended to control large page allocation >>>>>>> only for tmpfs mounts. Therefore, I find the current code confusing and >>>>>>> think it needs to be fixed: >>>>>>> >>>>>>> /* Don't consider 'deny' for emergencies and 'force' for testing */ >>>>>>> if (sb != shm_mnt->mnt_sb && sbinfo->huge) >>>>>>> mapping_set_large_folios(inode->i_mapping); >>>>>> >>> >>> Hi Baolin, >>> >>> Do you want to send a fix for this? >>> >>> Also I wonder how I can distinguish between anonymous shmem code and tmpfs code. >>> I thought they are the same thing except that they have different user interface, >>> but it seems that I was wrong. Sure. I can send a patch to make the code clear.