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Tue, 25 Feb 2025 07:34:34 -0800 (PST) X-Google-Smtp-Source: AGHT+IEbMcm+vXdAzJTYon6zBTCbwNNo7iLyCqaKKzChzEKHzoLL2wRRpT+N55nRj3Wf6bE/BOWkyQ== X-Received: by 2002:a05:6830:488c:b0:727:2f0c:916a with SMTP id 46e09a7af769-7274c1e2876mr13305738a34.16.1740497673722; Tue, 25 Feb 2025 07:34:33 -0800 (PST) Received: from x1.local ([2604:7a40:2041:2b00::1000]) by smtp.gmail.com with ESMTPSA id 46e09a7af769-7289df4eb9esm327787a34.42.2025.02.25.07.34.31 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 25 Feb 2025 07:34:33 -0800 (PST) Date: Tue, 25 Feb 2025 10:34:28 -0500 From: Peter Xu To: Barry Song <21cnbao@gmail.com> Cc: surenb@google.com, Liam.Howlett@oracle.com, aarcange@redhat.com, akpm@linux-foundation.org, axelrasmussen@google.com, bgeffon@google.com, brauner@kernel.org, david@redhat.com, hughd@google.com, jannh@google.com, kaleshsingh@google.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, lokeshgidra@google.com, mhocko@suse.com, ngeoffray@google.com, rppt@kernel.org, ryan.roberts@arm.com, shuah@kernel.org, v-songbaohua@oppo.com, viro@zeniv.linux.org.uk, willy@infradead.org, yuzhao@google.com, zhangpeng362@huawei.com, zhengtangquan@oppo.com Subject: Re: [PATCH RFC] mm: Fix kernel BUG when userfaultfd_move encounters swapcache Message-ID: References: <20250225110525.26732-1-21cnbao@gmail.com> MIME-Version: 1.0 In-Reply-To: <20250225110525.26732-1-21cnbao@gmail.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: ffHtax8w0Sya2PbgIB57pus-ihe1vJCrQO4sLSeC67o_1740497674 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 2E6BAC0020 X-Stat-Signature: 66i6rnmjjpbh3515iy8ir4r76qpkxeyh X-Rspam-User: X-HE-Tag: 1740497677-623957 X-HE-Meta: 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 lKIEu0kM 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Feb 26, 2025 at 12:05:25AM +1300, Barry Song wrote: > On Fri, Feb 21, 2025 at 1:36 PM Suren Baghdasaryan wrote: > > > > On Thu, Feb 20, 2025 at 3:52 PM Suren Baghdasaryan wrote: > > > > > > On Thu, Feb 20, 2025 at 3:47 PM Suren Baghdasaryan wrote: > > > > > > > > On Thu, Feb 20, 2025 at 2:59 PM Peter Xu wrote: > > > > > > > > > > On Thu, Feb 20, 2025 at 12:04:40PM +1300, Barry Song wrote: > > > > > > On Thu, Feb 20, 2025 at 11:15 AM Peter Xu wrote: > > > > > > > > > > > > > > On Thu, Feb 20, 2025 at 09:37:50AM +1300, Barry Song wrote: > > > > > > > > On Thu, Feb 20, 2025 at 7:27 AM Suren Baghdasaryan wrote: > > > > > > > > > > > > > > > > > > On Wed, Feb 19, 2025 at 3:25 AM Barry Song <21cnbao@gmail.com> wrote: > > > > > > > > > > > > > > > > > > > > From: Barry Song > > > > > > > > > > > > > > > > > > > > userfaultfd_move() checks whether the PTE entry is present or a > > > > > > > > > > swap entry. > > > > > > > > > > > > > > > > > > > > - If the PTE entry is present, move_present_pte() handles folio > > > > > > > > > > migration by setting: > > > > > > > > > > > > > > > > > > > > src_folio->index = linear_page_index(dst_vma, dst_addr); > > > > > > > > > > > > > > > > > > > > - If the PTE entry is a swap entry, move_swap_pte() simply copies > > > > > > > > > > the PTE to the new dst_addr. > > > > > > > > > > > > > > > > > > > > This approach is incorrect because even if the PTE is a swap > > > > > > > > > > entry, it can still reference a folio that remains in the swap > > > > > > > > > > cache. > > > > > > > > > > > > > > > > > > > > If do_swap_page() is triggered, it may locate the folio in the > > > > > > > > > > swap cache. However, during add_rmap operations, a kernel panic > > > > > > > > > > can occur due to: > > > > > > > > > > page_pgoff(folio, page) != linear_page_index(vma, address) > > > > > > > > > > > > > > > > > > Thanks for the report and reproducer! > > > > > > > > > > > > > > > > > > > > > > > > > > > > > $./a.out > /dev/null > > > > > > > > > > [ 13.336953] page: refcount:6 mapcount:1 mapping:00000000f43db19c index:0xffffaf150 pfn:0x4667c > > > > > > > > > > [ 13.337520] head: order:2 mapcount:1 entire_mapcount:0 nr_pages_mapped:1 pincount:0 > > > > > > > > > > [ 13.337716] memcg:ffff00000405f000 > > > > > > > > > > [ 13.337849] anon flags: 0x3fffc0000020459(locked|uptodate|dirty|owner_priv_1|head|swapbacked|node=0|zone=0|lastcpupid=0xffff) > > > > > > > > > > [ 13.338630] raw: 03fffc0000020459 ffff80008507b538 ffff80008507b538 ffff000006260361 > > > > > > > > > > [ 13.338831] raw: 0000000ffffaf150 0000000000004000 0000000600000000 ffff00000405f000 > > > > > > > > > > [ 13.339031] head: 03fffc0000020459 ffff80008507b538 ffff80008507b538 ffff000006260361 > > > > > > > > > > [ 13.339204] head: 0000000ffffaf150 0000000000004000 0000000600000000 ffff00000405f000 > > > > > > > > > > [ 13.339375] head: 03fffc0000000202 fffffdffc0199f01 ffffffff00000000 0000000000000001 > > > > > > > > > > [ 13.339546] head: 0000000000000004 0000000000000000 00000000ffffffff 0000000000000000 > > > > > > > > > > [ 13.339736] page dumped because: VM_BUG_ON_PAGE(page_pgoff(folio, page) != linear_page_index(vma, address)) > > > > > > > > > > [ 13.340190] ------------[ cut here ]------------ > > > > > > > > > > [ 13.340316] kernel BUG at mm/rmap.c:1380! > > > > > > > > > > [ 13.340683] Internal error: Oops - BUG: 00000000f2000800 [#1] PREEMPT SMP > > > > > > > > > > [ 13.340969] Modules linked in: > > > > > > > > > > [ 13.341257] CPU: 1 UID: 0 PID: 107 Comm: a.out Not tainted 6.14.0-rc3-gcf42737e247a-dirty #299 > > > > > > > > > > [ 13.341470] Hardware name: linux,dummy-virt (DT) > > > > > > > > > > [ 13.341671] pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--) > > > > > > > > > > [ 13.341815] pc : __page_check_anon_rmap+0xa0/0xb0 > > > > > > > > > > [ 13.341920] lr : __page_check_anon_rmap+0xa0/0xb0 > > > > > > > > > > [ 13.342018] sp : ffff80008752bb20 > > > > > > > > > > [ 13.342093] x29: ffff80008752bb20 x28: fffffdffc0199f00 x27: 0000000000000001 > > > > > > > > > > [ 13.342404] x26: 0000000000000000 x25: 0000000000000001 x24: 0000000000000001 > > > > > > > > > > [ 13.342575] x23: 0000ffffaf0d0000 x22: 0000ffffaf0d0000 x21: fffffdffc0199f00 > > > > > > > > > > [ 13.342731] x20: fffffdffc0199f00 x19: ffff000006210700 x18: 00000000ffffffff > > > > > > > > > > [ 13.342881] x17: 6c203d2120296567 x16: 6170202c6f696c6f x15: 662866666f67705f > > > > > > > > > > [ 13.343033] x14: 6567617028454741 x13: 2929737365726464 x12: ffff800083728ab0 > > > > > > > > > > [ 13.343183] x11: ffff800082996bf8 x10: 0000000000000fd7 x9 : ffff80008011bc40 > > > > > > > > > > [ 13.343351] x8 : 0000000000017fe8 x7 : 00000000fffff000 x6 : ffff8000829eebf8 > > > > > > > > > > [ 13.343498] x5 : c0000000fffff000 x4 : 0000000000000000 x3 : 0000000000000000 > > > > > > > > > > [ 13.343645] x2 : 0000000000000000 x1 : ffff0000062db980 x0 : 000000000000005f > > > > > > > > > > [ 13.343876] Call trace: > > > > > > > > > > [ 13.344045] __page_check_anon_rmap+0xa0/0xb0 (P) > > > > > > > > > > [ 13.344234] folio_add_anon_rmap_ptes+0x22c/0x320 > > > > > > > > > > [ 13.344333] do_swap_page+0x1060/0x1400 > > > > > > > > > > [ 13.344417] __handle_mm_fault+0x61c/0xbc8 > > > > > > > > > > [ 13.344504] handle_mm_fault+0xd8/0x2e8 > > > > > > > > > > [ 13.344586] do_page_fault+0x20c/0x770 > > > > > > > > > > [ 13.344673] do_translation_fault+0xb4/0xf0 > > > > > > > > > > [ 13.344759] do_mem_abort+0x48/0xa0 > > > > > > > > > > [ 13.344842] el0_da+0x58/0x130 > > > > > > > > > > [ 13.344914] el0t_64_sync_handler+0xc4/0x138 > > > > > > > > > > [ 13.345002] el0t_64_sync+0x1ac/0x1b0 > > > > > > > > > > [ 13.345208] Code: aa1503e0 f000f801 910f6021 97ff5779 (d4210000) > > > > > > > > > > [ 13.345504] ---[ end trace 0000000000000000 ]--- > > > > > > > > > > [ 13.345715] note: a.out[107] exited with irqs disabled > > > > > > > > > > [ 13.345954] note: a.out[107] exited with preempt_count 2 > > > > > > > > > > > > > > > > > > > > Fully fixing it would be quite complex, requiring similar handling > > > > > > > > > > of folios as done in move_present_pte. > > > > > > > > > > > > > > > > > > How complex would that be? Is it a matter of adding > > > > > > > > > folio_maybe_dma_pinned() checks, doing folio_move_anon_rmap() and > > > > > > > > > folio->index = linear_page_index like in move_present_pte() or > > > > > > > > > something more? > > > > > > > > > > > > > > > > My main concern is still with large folios that require a split_folio() > > > > > > > > during move_pages(), as the entire folio shares the same index and > > > > > > > > anon_vma. However, userfaultfd_move() moves pages individually, > > > > > > > > making a split necessary. > > > > > > > > > > > > > > > > However, in split_huge_page_to_list_to_order(), there is a: > > > > > > > > > > > > > > > > if (folio_test_writeback(folio)) > > > > > > > > return -EBUSY; > > > > > > > > > > > > > > > > This is likely true for swapcache, right? However, even for move_present_pte(), > > > > > > > > it simply returns -EBUSY: > > > > > > > > > > > > > > > > move_pages_pte() > > > > > > > > { > > > > > > > > /* at this point we have src_folio locked */ > > > > > > > > if (folio_test_large(src_folio)) { > > > > > > > > /* split_folio() can block */ > > > > > > > > pte_unmap(&orig_src_pte); > > > > > > > > pte_unmap(&orig_dst_pte); > > > > > > > > src_pte = dst_pte = NULL; > > > > > > > > err = split_folio(src_folio); > > > > > > > > if (err) > > > > > > > > goto out; > > > > > > > > > > > > > > > > /* have to reacquire the folio after it got split */ > > > > > > > > folio_unlock(src_folio); > > > > > > > > folio_put(src_folio); > > > > > > > > src_folio = NULL; > > > > > > > > goto retry; > > > > > > > > } > > > > > > > > } > > > > > > > > > > > > > > > > Do we need a folio_wait_writeback() before calling split_folio()? > > > > > > > > > > > > > > Maybe no need in the first version to fix the immediate bug? > > > > > > > > > > > > > > It's also not always the case to hit writeback here. IIUC, writeback only > > > > > > > happens for a short window when the folio was just added into swapcache. > > > > > > > MOVE can happen much later after that anytime before a swapin. My > > > > > > > understanding is that's also what Matthew wanted to point out. It may be > > > > > > > better justified of that in a separate change with some performance > > > > > > > measurements. > > > > > > > > > > > > The bug we’re discussing occurs precisely within the short window you > > > > > > mentioned. > > > > > > > > > > > > 1. add_to_swap: The folio is added to swapcache. > > > > > > 2. try_to_unmap: PTEs are converted to swap entries. > > > > > > 3. pageout > > > > > > 4. Swapcache is cleared. > > > > > > > > > > Hmm, I see. I was expecting step 4 to be "writeback is cleared".. or at > > > > > least that should be step 3.5, as IIUC "writeback" needs to be cleared > > > > > before "swapcache" bit being cleared. > > > > > > > > > > > > > > > > > The issue happens between steps 2 and 4, where the PTE is not present, but > > > > > > the folio is still in swapcache - the current code does move_swap_pte() but does > > > > > > not fixup folio->index within swapcache. > > > > > > > > > > One thing I'm still not clear here is why it's a race condition, rather > > > > > than more severe than that. I mean, folio->index is definitely wrong, then > > > > > as long as the page still in swapcache, we should be able to move the swp > > > > > entry over to dest addr of UFFDIO_MOVE, read on dest addr, then it'll see > > > > > the page in swapcache with the wrong folio->index already and trigger. > > > > > > > > > > I wrote a quick test like that, it actually won't trigger.. > > > > > > > > > > I had a closer look in the code, I think it's because do_swap_page() has > > > > > the logic to detect folio->index matching first, and allocate a new folio > > > > > if it doesn't match in ksm_might_need_to_copy(). IIUC that was for > > > > > ksm.. but it looks like it's functioning too here. > > > > > > > > > > ksm_might_need_to_copy: > > > > > if (folio_test_ksm(folio)) { > > > > > if (folio_stable_node(folio) && > > > > > !(ksm_run & KSM_RUN_UNMERGE)) > > > > > return folio; /* no need to copy it */ > > > > > } else if (!anon_vma) { > > > > > return folio; /* no need to copy it */ > > > > > } else if (folio->index == linear_page_index(vma, addr) && <---------- [1] > > > > > anon_vma->root == vma->anon_vma->root) { > > > > > return folio; /* still no need to copy it */ > > > > > } > > > > > ... > > > > > > > > > > new_folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0, vma, addr); <---- [2] > > > > > ... > > > > > > > > > > So I believe what I hit is at [1] it sees index doesn't match, then it > > > > > decided to allocate a new folio. In this case, it won't hit your BUG > > > > > because it'll be "folio != swapcache" later, so it'll setup the > > > > > folio->index for the new one, rather than the sanity check. > > > > > > > > > > Do you know how your case got triggered, being able to bypass the above [1] > > > > > which should check folio->index already? > > > > > > > > To understand the change I tried applying the proposed patch to both > > > > mm-unstable and Linus' ToT and got conflicts for both trees. Barry, > > > > which baseline are you using? > > > > > > Oops, never mind. My mistake. Copying from the email messed up tabs... > > > It applies cleanly. > > > > Overall the code seems correct to me, however the new code has quite > > complex logical structure IMO. Original simplified code structure is > > like this: > > > > if (pte_present(orig_src_pte)) { > > if (is_zero_pfn) { > > move_zeropage_pte() > > return > > } > > // pin and lock src_folio > > spin_lock(src_ptl) > > folio_get(folio) > > folio_trylock(folio) > > if (folio_test_large(src_folio)) > > split_folio(src_folio) > > anon_vma_trylock_write(src_anon_vma) > > move_present_pte() > > } else { > > if (non_swap_entry(entry)) > > if (is_migration_entry(entry)) > > handle migration entry > > else > > move_swap_pte() > > } > > > > The new structure looks like this: > > > > if (!pte_present(orig_src_pte)) { > > if (is_migration_entry(entry)) { > > handle migration entry > > return > > } > > if (!non_swap_entry() || !pte_swp_exclusive()) > > return > > si = get_swap_device(entry); > > } > > if (pte_present(orig_src_pte) && is_zero_pfn(pte_pfn(orig_src_pte))) > > move_zeropage_pte() > > return > > } > > pin and lock src_folio > > spin_lock(src_ptl) > > if (pte_present(orig_src_pte)) > > folio_get(folio) > > else { > > folio = filemap_get_folio(swap_entry) > > if (IS_ERR(folio)) > > move_swap_pte() > > return > > } > > } > > folio_trylock(folio) > > if (folio_test_large(src_folio)) > > split_folio(src_folio) > > if (pte_present(orig_src_pte)) > > anon_vma_trylock_write(src_anon_vma) > > move_pte_and_folio() > > > > This looks more complex and harder to follow. Might be the reason > > David was not in favour of treating swapcache and present pages in the > > same path. And now I would agree that refactoring some common parts > > and not breaking the original structure might be cleaner. > > Exactly, that’s the cost we’re facing in trying to share the code path > for swap and present PTEs. > > I tried to extract some common functions for present PTE and swap entries, > but I found too many detailed differences and variants. This made the common > function overly complex, turning it into a real "monster." As a result, I > don't think this approach would make the code any more readable or cleaner. > > After trying a couple of times, I feel the following is somehow more > readable: > (Lokesh is eager for the small folios fixes to be merged without further > delay. So, I'd prefer to return -EBUSY for large folios in the hotfixes > and handle the mTHP -EBUSY issue in a separate patch later.) > > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c > index 867898c4e30b..eed9286ec1f3 100644 > --- a/mm/userfaultfd.c > +++ b/mm/userfaultfd.c > @@ -18,6 +18,7 @@ > #include > #include > #include "internal.h" > +#include "swap.h" > > static __always_inline > bool validate_dst_vma(struct vm_area_struct *dst_vma, unsigned long dst_end) > @@ -1072,15 +1073,15 @@ static int move_present_pte(struct mm_struct *mm, > return err; > } > > -static int move_swap_pte(struct mm_struct *mm, > +static int move_swap_pte(struct mm_struct *mm, struct vm_area_struct *dst_vma, > unsigned long dst_addr, unsigned long src_addr, > pte_t *dst_pte, pte_t *src_pte, > pte_t orig_dst_pte, pte_t orig_src_pte, > pmd_t *dst_pmd, pmd_t dst_pmdval, > - spinlock_t *dst_ptl, spinlock_t *src_ptl) > + spinlock_t *dst_ptl, spinlock_t *src_ptl, > + struct folio *src_folio) > { > - if (!pte_swp_exclusive(orig_src_pte)) > - return -EBUSY; > + int err = 0; > > double_pt_lock(dst_ptl, src_ptl); > > @@ -1090,11 +1091,22 @@ static int move_swap_pte(struct mm_struct *mm, > return -EAGAIN; > } > > + if (src_folio) { We'd better add a comment here explaining src_folio in this case is a swap cache folio, and we're updating index to make sure if the folio can be reused later in a swapin, the rmap info will match, or something like that. > + /* Folio got pinned from under us. Put it back and fail the move. */ > + if (folio_maybe_dma_pinned(src_folio)) { > + err = -EBUSY; > + goto out; > + } If the swap entry is guaranteed exclusive (which I think will hold true), I think we can drop this and the "out" label, as it can't happen. > + folio_move_anon_rmap(src_folio, dst_vma); > + src_folio->index = linear_page_index(dst_vma, dst_addr); > + } > + > orig_src_pte = ptep_get_and_clear(mm, src_addr, src_pte); > set_pte_at(mm, dst_addr, dst_pte, orig_src_pte); > - double_pt_unlock(dst_ptl, src_ptl); > > - return 0; > +out: > + double_pt_unlock(dst_ptl, src_ptl); > + return err; > } > > static int move_zeropage_pte(struct mm_struct *mm, > @@ -1137,6 +1149,7 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, > __u64 mode) > { > swp_entry_t entry; > + struct swap_info_struct *si = NULL; > pte_t orig_src_pte, orig_dst_pte; > pte_t src_folio_pte; > spinlock_t *src_ptl, *dst_ptl; > @@ -1318,6 +1331,8 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, > orig_dst_pte, orig_src_pte, dst_pmd, > dst_pmdval, dst_ptl, src_ptl, src_folio); > } else { > + struct folio *folio = NULL; > + > entry = pte_to_swp_entry(orig_src_pte); > if (non_swap_entry(entry)) { > if (is_migration_entry(entry)) { > @@ -1331,9 +1346,47 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, > goto out; > } > > - err = move_swap_pte(mm, dst_addr, src_addr, dst_pte, src_pte, > - orig_dst_pte, orig_src_pte, dst_pmd, > - dst_pmdval, dst_ptl, src_ptl); > + if (!pte_swp_exclusive(orig_src_pte)) { > + err = -EBUSY; > + goto out; > + } > + > + si = get_swap_device(entry); > + if (unlikely(!si)) { > + err = -EAGAIN; > + goto out; > + } > + /* > + * Check if swapcache exists. If it does, the folio must be > + * moved even if the PTE is a swap entry. For large folios, > + * we directly return -EBUSY, as split_folio() currently > + * also returns -EBUSY when attempting to split unmapped > + * large folios in the swapcache. This needs to be fixed > + * to allow proper handling. > + */ Some alignment issue on comments... We could also add something on the decision on why not taking anon_vma. IIUC mention that no possible rmap walker when exclusive and unmapped should be ok as of now.. > + if (!src_folio) > + folio = filemap_get_folio(swap_address_space(entry), > + swap_cache_index(entry)); > + if (!IS_ERR_OR_NULL(folio)) { > + if (folio_test_large(folio)) { > + err = -EBUSY; > + folio_put(folio); > + goto out; > + } > + src_folio = folio; We need to update src_folio_pte here, or later it might access uninitialized stack var when a retry needed: if (src_folio && unlikely(!pte_same(src_folio_pte, orig_src_pte))) { err = -EAGAIN; goto out; } > + if (!folio_trylock(src_folio)) { > + pte_unmap(&orig_src_pte); > + pte_unmap(&orig_dst_pte); > + src_pte = dst_pte = NULL; > + /* now we can block and wait */ > + folio_lock(src_folio); > + si = NULL; Swap device ref leak? Thanks, > + goto retry; > + } > + } > + err = move_swap_pte(mm, dst_vma, dst_addr, src_addr, dst_pte, src_pte, > + orig_dst_pte, orig_src_pte, dst_pmd, dst_pmdval, > + dst_ptl, src_ptl, src_folio); > } > > out: > @@ -1350,6 +1403,8 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, > if (src_pte) > pte_unmap(src_pte); > mmu_notifier_invalidate_range_end(&range); > + if (si) > + put_swap_device(si); > > return err; > } > > If there are no objections, I'll send v2 tomorrow with the above code. > 12:04 AM, Time to get some sleep now! :-) > > > > > > > > > > > > > > > > > > > > > > > > > > > > My point is that if we want a proper fix for mTHP, we'd better handle writeback. > > > > > > Otherwise, this isn’t much different from directly returning -EBUSY as proposed > > > > > > in this RFC. > > > > > > > > > > > > For small folios, there’s no split_folio issue, making it relatively > > > > > > simpler. Lokesh > > > > > > mentioned plans to madvise NOHUGEPAGE in ART, so fixing small folios is likely > > > > > > the first priority. > > > > > > > > > > Agreed. > > > > > > > > > > -- > > > > > Peter Xu > > > > > > > Thanks > Barry > > -- Peter Xu