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charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: 7F86118000A X-Stat-Signature: 7h1dsmty3sew8khbkh1exbxfu6je9j14 X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1754405850-155956 X-HE-Meta: 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 LrTXxC0g L9SpqNAPEMuhtNgnE2Nu7BT43X+X4HtTIG9w3kS/QF0Q6DrBd91mUf8ynTJsnX7m4rJJFH99QmZZGBxs9tyHUigOKJ3WuM0EWNQDQgN2uumEDeglcoIqGWjeDR93w9+B3OXLUahrZ8zXcHLymuVtybMr9h0VYSqfW6WofJ5B/maDYEQOcOiR0KK8D1Mbg5mTRA9YxHrLF18XMGHbjA2R5kU0PssfqKfGC8aDcS8thvnUG3PsI29TV44+sRz770Ssmzhf7FzSCuuSY1rtRs9jNNYRisckad1uKxRvyUPDsQ5hv7Cv6tmFwGIyjtyAWK018IHor6902Hfa9hh/GOzAxtxf4EBSJYZLZN4NSKYG++FevoyPAAiNjXL4xr4ahcu/vPuk6aQtTtjEEdjzGZungioamLWO0jH6Tqo/fDZZ0cKyJljp5O99+mbzUplimsm+6JyGSb97tAqXxI00DZ0loaYofM1xV2hvJa/iVtp0Qhfoebok= 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 Tue, Aug 5, 2025 at 7:39=E2=80=AFAM Peter Xu wrote: > > On Mon, Aug 04, 2025 at 07:55:42AM -0700, Suren Baghdasaryan wrote: > > On Fri, Aug 1, 2025 at 5:32=E2=80=AFPM Peter Xu wro= te: > > > > > > On Fri, Aug 01, 2025 at 07:30:02PM +0000, Suren Baghdasaryan wrote: > > > > On Fri, Aug 1, 2025 at 6:21=E2=80=AFPM Peter Xu = wrote: > > > > > > > > > > On Fri, Aug 01, 2025 at 05:45:10PM +0000, Suren Baghdasaryan wrot= e: > > > > > > On Fri, Aug 1, 2025 at 5:13=E2=80=AFPM Peter Xu wrote: > > > > > > > > > > > > > > On Fri, Aug 01, 2025 at 09:41:31AM -0700, Suren Baghdasaryan = wrote: > > > > > > > > On Fri, Aug 1, 2025 at 9:23=E2=80=AFAM Peter Xu wrote: > > > > > > > > > > > > > > > > > > On Fri, Aug 01, 2025 at 08:28:38AM -0700, Suren Baghdasar= yan wrote: > > > > > > > > > > On Fri, Aug 1, 2025 at 7:16=E2=80=AFAM Peter Xu wrote: > > > > > > > > > > > > > > > > > > > > > > On Fri, Aug 01, 2025 at 09:21:30AM +0200, David Hilde= nbrand wrote: > > > > > > > > > > > > On 31.07.25 17:44, Suren Baghdasaryan wrote: > > > > > > > > > > > > > > > > > > > > > > > > Hi! > > > > > > > > > > > > > > > > > > > > > > > > Did you mean in you patch description: > > > > > > > > > > > > > > > > > > > > > > > > "userfaultfd: fix a crash in UFFDIO_MOVE with some = non-present PMDs" > > > > > > > > > > > > > > > > > > > > > > > > Talking about THP holes is very very confusing. > > > > > > > > > > > > > > > > > > > > > > > > > When UFFDIO_MOVE is used with UFFDIO_MOVE_MODE_AL= LOW_SRC_HOLES and it > > > > > > > > > > > > > encounters a non-present THP, it fails to properl= y recognize an unmapped > > > > > > > > > > > > > > > > > > > > > > > > You mean a "non-present PMD that is not a migration= entry". > > > > > > > > > > > > > > > > > > > > > > > > > hole and tries to access a non-existent folio, re= sulting in > > > > > > > > > > > > > a crash. Add a check to skip non-present THPs. > > > > > > > > > > > > > > > > > > > > > > > > That makes sense. The code we have after this patch= is rather complicated > > > > > > > > > > > > and hard to read. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uA= BI") > > > > > > > > > > > > > Reported-by: syzbot+b446dbe27035ef6bd6c2@syzkalle= r.appspotmail.com > > > > > > > > > > > > > Closes: https://lore.kernel.org/all/68794b5c.a70a= 0220.693ce.0050.GAE@google.com/ > > > > > > > > > > > > > Signed-off-by: Suren Baghdasaryan > > > > > > > > > > > > > Cc: stable@vger.kernel.org > > > > > > > > > > > > > --- > > > > > > > > > > > > > Changes since v1 [1] > > > > > > > > > > > > > - Fixed step size calculation, per Lokesh Gidra > > > > > > > > > > > > > - Added missing check for UFFDIO_MOVE_MODE_ALLOW_= SRC_HOLES, per Lokesh Gidra > > > > > > > > > > > > > > > > > > > > > > > > > > [1] https://lore.kernel.org/all/20250730170733.38= 29267-1-surenb@google.com/ > > > > > > > > > > > > > > > > > > > > > > > > > > mm/userfaultfd.c | 45 +++++++++++++++++++++++++= ++++---------------- > > > > > > > > > > > > > 1 file changed, 29 insertions(+), 16 deletions(= -) > > > > > > > > > > > > > > > > > > > > > > > > > > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c > > > > > > > > > > > > > index cbed91b09640..b5af31c22731 100644 > > > > > > > > > > > > > --- a/mm/userfaultfd.c > > > > > > > > > > > > > +++ b/mm/userfaultfd.c > > > > > > > > > > > > > @@ -1818,28 +1818,41 @@ ssize_t move_pages(struct= userfaultfd_ctx *ctx, unsigned long dst_start, > > > > > > > > > > > > > ptl =3D pmd_trans_huge_lock(src_pmd, = src_vma); > > > > > > > > > > > > > if (ptl) { > > > > > > > > > > > > > - /* Check if we can move the p= md without splitting it. */ > > > > > > > > > > > > > - if (move_splits_huge_pmd(dst_= addr, src_addr, src_start + len) || > > > > > > > > > > > > > - !pmd_none(dst_pmdval)) { > > > > > > > > > > > > > - struct folio *folio = =3D pmd_folio(*src_pmd); > > > > > > > > > > > > > + if (pmd_present(*src_pmd) || = is_pmd_migration_entry(*src_pmd)) { > > > > > > > > > > > > > > > > > > > > > > [1] > > > > > > > > > > > > > > > > > > > > > > > > + /* Check if we can mo= ve the pmd without splitting it. */ > > > > > > > > > > > > > + if (move_splits_huge_= pmd(dst_addr, src_addr, src_start + len) || > > > > > > > > > > > > > + !pmd_none(dst_pmd= val)) { > > > > > > > > > > > > > + if (pmd_prese= nt(*src_pmd)) { > > > > > > > > > > > > > > > > > > [2] > > > > > > > > > > > > > > > > > > > > > > + struc= t folio *folio =3D pmd_folio(*src_pmd); > > > > > > > > > > > > > > > > > > [3] > > > > > > > > > > > > > > > > > > > > > > + > > > > > > > > > > > > > + if (!= folio || (!is_huge_zero_folio(folio) && > > > > > > > > > > > > > + = !PageAnonExclusive(&folio->page))) { > > > > > > > > > > > > > + = spin_unlock(ptl); > > > > > > > > > > > > > + = err =3D -EBUSY; > > > > > > > > > > > > > + = break; > > > > > > > > > > > > > + } > > > > > > > > > > > > > + } > > > > > > > > > > > > > > > > > > > > > > > > ... in particular that. Is there some way to make t= his code simpler / easier > > > > > > > > > > > > to read? Like moving that whole last folio-check th= ingy into a helper? > > > > > > > > > > > > > > > > > > > > > > One question might be relevant is, whether the check = above [1] can be > > > > > > > > > > > dropped. > > > > > > > > > > > > > > > > > > > > > > The thing is __pmd_trans_huge_lock() does double chec= k the pmd to be !none > > > > > > > > > > > before returning the ptl. I didn't follow closely on= the recent changes on > > > > > > > > > > > mm side on possible new pmd swap entries, if migratio= n is the only possible > > > > > > > > > > > one then it looks like [1] can be avoided. > > > > > > > > > > > > > > > > > > > > Hi Peter, > > > > > > > > > > is_swap_pmd() check in __pmd_trans_huge_lock() allows f= or (!pmd_none() > > > > > > > > > > && !pmd_present()) PMD to pass and that's when this cra= sh is hit. > > > > > > > > > > > > > > > > > > First for all, thanks for looking into the issue with Lok= esh; I am still > > > > > > > > > catching up with emails after taking weeks off. > > > > > > > > > > > > > > > > > > I didn't yet read into the syzbot report, but I thought t= he bug was about > > > > > > > > > referencing the folio on top of a swap entry after readin= g your current > > > > > > > > > patch, which has: > > > > > > > > > > > > > > > > > > if (move_splits_huge_pmd(dst_addr, src_addr, src_= start + len) || > > > > > > > > > !pmd_none(dst_pmdval)) { > > > > > > > > > struct folio *folio =3D pmd_folio(*src_pm= d); <---- > > > > > > > > > > > > > > > > > > Here looks like *src_pmd can be a migration entry. Is my = understanding > > > > > > > > > correct? > > > > > > > > > > > > > > > > Correct. > > > > > > > > > > > > > > > > > > > > > > > > > > > If we drop the check at [1] then the path that takes us= to > > > > > > > > > > > > > > > > > > If my above understanding is correct, IMHO it should be [= 2] above that > > > > > > > > > makes sure the reference won't happen on a swap entry, no= t necessarily [1]? > > > > > > > > > > > > > > > > Yes, in case of migration entry this is what protects us. > > > > > > > > > > > > > > > > > > > > > > > > > > > split_huge_pmd() will bail out inside split_huge_pmd_lo= cked() with no > > > > > > > > > > indication that split did not happen. Afterwards we wil= l retry > > > > > > > > > > > > > > > > > > So we're talking about the case where it's a swap pmd ent= ry, right? > > > > > > > > > > > > > > > > Hmm, my understanding is that it's being treated as a swap = entry but > > > > > > > > in reality is not. I thought THPs are always split before t= hey get > > > > > > > > swapped, no? > > > > > > > > > > > > > > Yes they should be split, afaiu. > > > > > > > > > > > > > > > > > > > > > > > > Could you elaborate why the split would fail? > > > > > > > > > > > > > > > > Just looking at the code, split_huge_pmd_locked() checks fo= r > > > > > > > > (pmd_trans_huge(*pmd) || is_pmd_migration_entry(*pmd)). > > > > > > > > pmd_trans_huge() is false if !pmd_present() and it's not a = migration > > > > > > > > entry, so __split_huge_pmd_locked() will be skipped. > > > > > > > > > > > > > > Here might be the major part of where confusion came from: I = thought it > > > > > > > must be a migration pmd entry to hit the issue, so it's not? > > > > > > > > > > > > > > I checked the code just now: > > > > > > > > > > > > > > __handle_mm_fault: > > > > > > > if (unlikely(is_swap_pmd(vmf.orig_pmd))) { > > > > > > > VM_BUG_ON(thp_migration_supported() &= & > > > > > > > !is_pmd_migration_e= ntry(vmf.orig_pmd)); > > > > > > > > > > > > > > So IIUC pmd migration entry is still the only possible way to= have a swap > > > > > > > entry. It doesn't look like we have "real" swap entries for = PMD (which can > > > > > > > further points to some swapfiles)? > > > > > > > > > > > > Correct. AFAIU here we stumble on a pmd entry which was allocat= ed but > > > > > > never populated. > > > > > > > > > > Do you mean a pmd_none()? > > > > > > > > Yes. > > > > > > > > > > > > > > If so, that goes back to my original question, on why > > > > > __pmd_trans_huge_lock() returns non-NULL if it's a pmd_none()? I= MHO it > > > > > really should have returned NULL for pmd_none(). > > > > > > > > That was exactly the answer I gave Lokesh when he theorized about t= he > > > > cause of this crash but after reproducing it I saw that > > > > pmd_trans_huge_lock() happily returns the PTL as long as PMD is not > > > > pmd_none(). And that's because it passes as is_swap_pmd(). But even= if > > > > we change that we still need to implement the code to skip the enti= re > > > > PMD. > > > > > > The thing is I thought if pmd_trans_huge_lock() can return non-NULL, = it > > > must be either a migration entry or a present THP. So are you describ= ing a > > > THP but with present bit cleared? Do you know what is that entry, an= d why > > > it has present bit cleared? > > > > In this case it's because earlier we allocated that PMD here: > > https://elixir.bootlin.com/linux/v6.16/source/mm/userfaultfd.c#L1797 > > AFAIU, this line is not about allocation of any pmd entry, but the pmd > pgtable page that _holds_ the PMDs: > > static inline pmd_t *pmd_alloc(struct mm_struct *mm, pud_t *pud, unsigned= long address) > { > return (unlikely(pud_none(*pud)) && __pmd_alloc(mm, pud, address)= )? > NULL: pmd_offset(pud, address); > } > > It makes sure the PUD entry, not the PMD entry, be populated. Hmm. Then I was reading this code completely wrong and need to rethink what is happening here. > > > but wouldn't that be the same if the PMD was mapped and then got > > unmapped later? My understanding is that we allocate the PMD at the > > line I pointed to make UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES case the same > > as this unmapped PMD case. If my assumption is incorrect then we could > > skip the hole earlier instead of allocating the PMD for it. > > > > > > > > I think my attention got attracted to pmd migration entry too much, s= o I > > > didn't really notice such possibility, as I believe migration pmd is = broken > > > already in this path. > > > > > > The original code: > > > > > > ptl =3D pmd_trans_huge_lock(src_pmd, src_vma); > > > if (ptl) { > > > /* Check if we can move the pmd without split= ting it. */ > > > if (move_splits_huge_pmd(dst_addr, src_addr, = src_start + len) || > > > !pmd_none(dst_pmdval)) { > > > struct folio *folio =3D pmd_folio(*sr= c_pmd); > > > > > > if (!folio || (!is_huge_zero_folio(fo= lio) && > > > !PageAnonExclusive(&fo= lio->page))) { > > > spin_unlock(ptl); > > > err =3D -EBUSY; > > > break; > > > } > > > > > > spin_unlock(ptl); > > > split_huge_pmd(src_vma, src_pmd, src_= addr); > > > /* The folio will be split by move_pa= ges_pte() */ > > > continue; > > > } > > > > > > err =3D move_pages_huge_pmd(mm, dst_pmd, src_= pmd, > > > dst_pmdval, dst_vma= , src_vma, > > > dst_addr, src_addr)= ; > > > step_size =3D HPAGE_PMD_SIZE; > > > } else { > > > > > > It'll get ptl for a migration pmd, then pmd_folio is risky without ch= ecking > > > present bit. That's what my previous smaller patch wanted to fix. > > > > > > But besides that, IIUC it's all fine at least for a pmd migration ent= ry, > > > because when with the smaller patch applied, either we'll try to spli= t the > > > pmd migration entry, or we'll do move_pages_huge_pmd(), which interna= lly > > > handles the pmd migration entry too by waiting on it: > > > > > > if (!pmd_trans_huge(src_pmdval)) { > > > spin_unlock(src_ptl); > > > if (is_pmd_migration_entry(src_pmdval)) { > > > pmd_migration_entry_wait(mm, &src_pmdval); > > > return -EAGAIN; > > > } > > > return -ENOENT; > > > } > > > > > > Then logically after the migration entry got recovered, we'll either = see a > > > real THP or pmd none next time. > > > > Yes, for migration entries adding the "if (pmd_present(*src_pmd))" > > before getting the folio is enough. The problematic case is > > (!pmd_none(*src_pmd) && !pmd_present(*src_pmd)) and not a migration > > entry. > > I thought we could have any of below here on the pmd entry: > > (0) pmd_none, which should constantly have pmd_trans_huge_lock -> NULL > > (1) pmd pgtable entry, which must have PRESENT && !TRANS, so > pmd_trans_huge_lock -> NULL, > > (2) pmd migration, pmd_trans_huge_lock -> valid > > (3) pmd thp, pmd_trans_huge_lock -> valid > > I thought (2) was broken, which we seem to agree upon.. however if so the > smaller patch should fix it, per explanation in my previous reply. OTOH = I > can't think of (4). The case I was hitting is (!pmd_none && !pmd_present && !is_pmd_migration_entry). My original thinking was that this entry was newly allocated at the line I pointed earlier but now I'm not so sure anymore. > > Said that, I just noticed (3) can be broken as well - could it be a > prot_none entry? The very confusing part of this patch is it seems to > think it's pmd_none() here as holes: > > if (pmd_present(*src_pmd) || is_pmd_migration_entry(*src_pmd)) { > ... > } else { > spin_unlock(ptl); > if (!(mode & UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES)) { > err =3D -ENOENT; > break; > } > /* nothing to do to move a hole */ > err =3D 0; > step_size =3D min(HPAGE_PMD_SIZE, src_start + len - src_a= ddr); > } > > But is it really? Again, I don't think pmd_none() could happen with > pmd_trans_huge_lock() returning the ptl. That is true, in the pmd_none() case pmd_trans_huge_lock() returns NULL. > > Could you double check this? E.g. with this line if that makes sense to > you: > > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c > index 8bf8ff0be990f..d2d4f2a0ae69f 100644 > --- a/mm/userfaultfd.c > +++ b/mm/userfaultfd.c > @@ -1903,6 +1903,7 @@ ssize_t move_pages(struct userfaultfd_ctx *ctx, uns= igned long dst_start, > dst_addr, src_a= ddr); > step_size =3D HPAGE_PMD_SIZE; > } else { > + BUG_ON(!pmd_none(*src_pmd)); > spin_unlock(ptl); > if (!(mode & UFFDIO_MOVE_MODE_ALLOW_SRC_H= OLES)) { > err =3D -ENOENT; > > I would expect it constantly BUG() here, if that explains my thoughts. I'll add this and check. > > Now I doubt it's a prot_none THP.. aka, a THP that got numa hint to be > moved. If so, we may need to process it / move it / .. but we likely > should never skip it. We can double check the buggy pmd entry you hit > (besides migration entry) first. Let me log the flags of the entry when this issue happens. That should provide more insights. Thanks, Suren. > > Thanks, > > -- > Peter Xu >