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Thu, 27 Aug 2026 07:51:49 +0000 (GMT) Received: from li-008a6a4c-3549-11b2-a85c-c5cc2836eea2.ibm.com (unknown [9.111.20.232]) by smtpav03.fra02v.mail.ibm.com (Postfix) with ESMTPS; Thu, 27 Aug 2026 07:51:48 +0000 (GMT) Date: Thu, 27 Aug 2026 09:51:47 +0200 From: Alexander Gordeev To: Heiko Carstens Cc: Gerald Schaefer , linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] s390/mm: Fix fail path in crst_table_upgrade() Message-ID: <8a959b1e-7b29-4539-a7d5-9c77cf397aed-agordeev@linux.ibm.com> References: <20260811121316.2622460-1-agordeev@linux.ibm.com> <20260811141203.12308B7d-hca@linux.ibm.com> <9e707e9c-2da5-4325-9bf3-b41aefb2f8ab-agordeev@linux.ibm.com> <20260824142227.11040D30-hca@linux.ibm.com> Precedence: bulk X-Mailing-List: linux-s390@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260824142227.11040D30-hca@linux.ibm.com> X-TM-AS-GCONF: 00 X-Proofpoint-Spam-Info: AW1haW4tMjYwODI3MDA2MSBTYWx0ZWRfX1rja2OnhlBgu v5kHvWryOBzM9jxiqlnozV5BCCsn3yR8bf3tZp4k1MASlAHAlDJxMMoXVBPjVf9mAv390SbcOda H2OZaRDAlFQDHWUs+XtibbBy+8RbjdM= X-Authority-Analysis: v=2.4 cv=AYuB2XXG c=1 sm=1 tr=0 ts=6a8fec9b cx=c_pps a=AfN7/Ok6k8XGzOShvHwTGQ==:117 a=AfN7/Ok6k8XGzOShvHwTGQ==:17 a=kj9zAlcOel0A:10 a=Sv0fKeRqtYgA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=RzCfie-kr_QcCd8fBx8p:22 a=VnNF1IyMAAAA:8 a=I2Cq1U3C2w4F9YOXbjMA:9 a=CjuIK1q_8ugA:10 X-Proofpoint-ORIG-GUID: yDIfzNw1EtebX1OL96VQ2VY1yr5yQPFB X-Proofpoint-GUID: yDIfzNw1EtebX1OL96VQ2VY1yr5yQPFB X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwODI3MDA2MSBTYWx0ZWRfX3FAgdjFv+kRF eB9J09FZy28Dkk8pqss2z50gKz6JnxXNJ5N+xtJ0jE8Tqtaw8YxNzIAZ407RO4363M8nR3jSZiP 14lrQ0WKnQ5CjdRaRuYh8yR3O66kWaz6tuFuJgwYlcQ5HfTWPtn0vQlvgrCenTQ5Th5ZtQ3tHiX b802b4Vt3ECkhsYm9OaTL8q3lnOWtmcCOT4O/gA+R3s07e4szfy/YlHDhnBydEc1PaDivHR8dOF HnXFKYJfuC0SKYXAE1MjqmpZPtWB6oYjhSnx998/XFxsss8OoWpbt7rFDFlM5IscbkHPYMUkwwR rJDj6wbRL+8BS9Wcz5/yEmMJu/HM2QvvkngBd0EA3Kt/709X392DS02MaU3rSqHWmeQn6+qTqdb B+PcWYrSns63zngNYU1A5rNmFVkl9QSEpbT3lkHOZA30Vu7iHEGFxE7bqwuSLRS62grJkDzUeSt /T/vgPzMmdLBwUbjf7w== X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1176,Hydra:6.1.134,FMLib:17.12.100.49 definitions=2026-08-27_03,2026-08-26_02,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 malwarescore=0 phishscore=0 clxscore=1015 adultscore=0 bulkscore=0 impostorscore=0 priorityscore=1501 lowpriorityscore=0 spamscore=0 suspectscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2606150000 definitions=main-2608270061 On Mon, Aug 24, 2026 at 04:22:27PM +0200, Heiko Carstens wrote: > On Wed, Aug 19, 2026 at 01:22:50PM +0200, Alexander Gordeev wrote: > > On Tue, Aug 11, 2026 at 04:12:03PM +0200, Heiko Carstens wrote: > > > Imho this makes the code even more uglier than it is/was. This is also yet > > > another bug which is/was only possible after loop unrolling happened with > > > [1]. Another bug, which would not have been possible without the rework back > > > then, is [2]. > > > > > > So I would _much_ rather prefer to go back to what we had with [1], of course > > > including the additional dtor/ctor code. But that code was much more readable > > > compared to what we have now, especially considering the additional fix which > > > is proposed now. > > > > Frankly, to me personally the old version is more difficult to read. > > In addition, in case of 3-5 upgrade on 4-5 allocation failure it has > > (a) 4-level table leak and > > Which leak? The old code never had leak where the allocated 4-level table > would become unreachable (== leaked). > > > (b) useless 3-4 upgrade. > > This was an intentional design choice to keep the code simple. > > > Should all of those get fixed, I doubt the end result will look as > > simple as the original version. But I can try if you insist. > > So "all of that" :) is at most b). But it really doesn't make any sense to do > a rollback, simply because all of this is completely irrelevant for production > kernels. Order-2 allocations can only fail with error injection, otherwise > they never fail. > > Hence the partial upgrade is fine - this is not a leak, but a partial proper > upgrade of the address space size of the current task, with proper freeing of > the page table (no leak) on process exit. > > Note: the below keeps the oddity that pagetable_p4d_ctor() is called in case > of only a 4-level upgrade, even though in such a case pagetable_pgd_ctor() > would be "more" correct. In a similar way we have the oddity that after a page > table upgrade on process exit for the third level pagetable_pmd_dtor() is > called, even though on process start pagetable_pgd_ctor() was called for that > level. The current common code doesn't care about this, but all of this > doesn't look nice. > > Anyway, the ctor/dtor issue is something different. The below would simplify > the code to what we had before the unrolling. Since your version is complete, would you like to post it? If yes, please find: Reviewed-by: Alexander Gordeev > arch/s390/mm/pgalloc.c | 89 +++++++++++++++++------------------------- > 1 file changed, 36 insertions(+), 53 deletions(-) > > diff --git a/arch/s390/mm/pgalloc.c b/arch/s390/mm/pgalloc.c > index 5a3745cf7330..787e4d5fabea 100644 > --- a/arch/s390/mm/pgalloc.c > +++ b/arch/s390/mm/pgalloc.c > @@ -53,63 +53,46 @@ static void __crst_table_upgrade(void *arg) > > int crst_table_upgrade(struct mm_struct *mm, unsigned long end) > { > - unsigned long *pgd = NULL, *p4d = NULL, *__pgd; > - unsigned long asce_limit = mm->context.asce_limit; > + unsigned long *table, *pgd; > + int rc, notify; > > mmap_assert_write_locked(mm); > - > /* upgrade should only happen from 3 to 4, 3 to 5, or 4 to 5 levels */ > - VM_BUG_ON(asce_limit < _REGION2_SIZE); > - > - if (end <= asce_limit) > - return 0; > - > - if (asce_limit == _REGION2_SIZE) { > - p4d = crst_table_alloc(mm); > - if (unlikely(!p4d)) > - goto err_p4d; > - crst_table_init(p4d, _REGION2_ENTRY_EMPTY); > - pagetable_p4d_ctor(virt_to_ptdesc(p4d)); > + VM_BUG_ON(mm->context.asce_limit < _REGION2_SIZE); > + rc = 0; > + notify = 0; > + while (mm->context.asce_limit < end) { > + table = crst_table_alloc(mm); > + if (!table) { > + rc = -ENOMEM; > + break; > + } > + spin_lock_bh(&mm->page_table_lock); > + pgd = (unsigned long *)mm->pgd; > + if (mm->context.asce_limit == _REGION2_SIZE) { > + crst_table_init(table, _REGION2_ENTRY_EMPTY); > + p4d_populate(mm, (p4d_t *)table, (pud_t *)pgd); > + pagetable_p4d_ctor(virt_to_ptdesc(table)); > + mm->pgd = (pgd_t *)table; > + mm->context.asce_limit = _REGION1_SIZE; > + mm->context.asce = __pa(mm->pgd) | _ASCE_TABLE_LENGTH | > + _ASCE_USER_BITS | _ASCE_TYPE_REGION2; > + mm_inc_nr_puds(mm); > + } else { > + crst_table_init(table, _REGION1_ENTRY_EMPTY); > + pgd_populate(mm, (pgd_t *)table, (p4d_t *)pgd); > + pagetable_pgd_ctor(virt_to_ptdesc(table)); > + mm->pgd = (pgd_t *)table; > + mm->context.asce_limit = TASK_SIZE_MAX; > + mm->context.asce = __pa(mm->pgd) | _ASCE_TABLE_LENGTH | > + _ASCE_USER_BITS | _ASCE_TYPE_REGION1; > + } > + notify = 1; > + spin_unlock_bh(&mm->page_table_lock); > } > - if (end > _REGION1_SIZE) { > - pgd = crst_table_alloc(mm); > - if (unlikely(!pgd)) > - goto err_pgd; > - crst_table_init(pgd, _REGION1_ENTRY_EMPTY); > - pagetable_pgd_ctor(virt_to_ptdesc(pgd)); > - } > - > - spin_lock_bh(&mm->page_table_lock); > - > - if (p4d) { > - __pgd = (unsigned long *) mm->pgd; > - p4d_populate(mm, (p4d_t *) p4d, (pud_t *) __pgd); > - mm->pgd = (pgd_t *) p4d; > - mm->context.asce_limit = _REGION1_SIZE; > - mm->context.asce = __pa(mm->pgd) | _ASCE_TABLE_LENGTH | > - _ASCE_USER_BITS | _ASCE_TYPE_REGION2; > - mm_inc_nr_puds(mm); > - } > - if (pgd) { > - __pgd = (unsigned long *) mm->pgd; > - pgd_populate(mm, (pgd_t *) pgd, (p4d_t *) __pgd); > - mm->pgd = (pgd_t *) pgd; > - mm->context.asce_limit = TASK_SIZE_MAX; > - mm->context.asce = __pa(mm->pgd) | _ASCE_TABLE_LENGTH | > - _ASCE_USER_BITS | _ASCE_TYPE_REGION1; > - } > - > - spin_unlock_bh(&mm->page_table_lock); > - > - on_each_cpu(__crst_table_upgrade, mm, 0); > - > - return 0; > - > -err_pgd: > - pagetable_dtor(virt_to_ptdesc(p4d)); > - crst_table_free(mm, p4d); > -err_p4d: > - return -ENOMEM; > + if (notify) > + on_each_cpu(__crst_table_upgrade, mm, 0); > + return rc; > } > > unsigned long *page_table_alloc_noprof(struct mm_struct *mm) > -- > 2.53.0 Thanks!