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Received: from willy by casper.infradead.org with local (Exim 4.98.2 #2 (Red Hat Linux)) id 1vOdP1-0000000Bsna-0xik; Thu, 27 Nov 2025 14:55:23 +0000 Date: Thu, 27 Nov 2025 14:55:23 +0000 From: Matthew Wilcox To: Christian =?iso-8859-1?Q?K=F6nig?= Cc: Jan Sokolowski , linux-kernel@vger.kernel.org, Andrew Morton , linux-fsdevel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [RFC PATCH 1/1] idr: do not create idr if new id would be outside given range Message-ID: References: <20251127092732.684959-1-jan.sokolowski@intel.com> <20251127092732.684959-2-jan.sokolowski@intel.com> <06dbd4f8-ef5f-458c-a8b4-8a8fb2a7877c@amd.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Queue-Id: EF1B9100002 X-Rspamd-Server: rspam02 X-Stat-Signature: pteyb1ssjmoawgstwbrh6bjchxt5jhen X-Rspam-User: X-HE-Tag: 1764255326-135322 X-HE-Meta: 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 XXVL+MLO BYGtewRWkNHfggCs2bjl7KQTcirkl8/jLNJu1Fwz9f4orJfz29V95cvq580hwB0nCupOjVN4XF4/o9ZAJPsV9Yvf3u/xUg/PBJFIjLtJoWvNSsTRmnGjwGKM+Rq5K3u0bR/R9rByCFBYuSmXEGNukhu1vsp5Ibge/zyw7Vlmd3C9zbmwjg/NNXrWKZr0kzCxVxd11Vo7fYvpPLYZ4GZlVxHnsmIzRF8jdrDfzz43hbhfd/o8d15T7JIrHvA== 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 Thu, Nov 27, 2025 at 02:11:02PM +0000, Matthew Wilcox wrote: > Hm. That's not what it does for me. It gives me id == 1, which isn't > correct! I'll look into that, but it'd be helpful to know what > combination of inputs gives us 2. Oh, never mind, I see what's happening. int idr_alloc(struct idr *idr, void *ptr, int start, int end, gfp_t gfp) ret = idr_alloc_u32(idr, ptr, &id, end > 0 ? end - 1 : INT_MAX, gfp); so it's passing 0 as 'max' to idr_alloc_u32() which does: slot = idr_get_free(&idr->idr_rt, &iter, gfp, max - base); and max - base becomes -1 or rather ULONG_MAX, and so we'll literally allocate any number. If the first slot is full, we'll get back 1 and then add 'base' to it, giving 2. Here's the new test-case: +void idr_alloc2_test(void) +{ + int id; + struct idr idr = IDR_INIT_BASE(idr, 1); + + id = idr_alloc(&idr, idr_alloc2_test, 0, 1, GFP_KERNEL); + assert(id == -ENOSPC); + + id = idr_alloc(&idr, idr_alloc2_test, 1, 2, GFP_KERNEL); + assert(id == 1); + + id = idr_alloc(&idr, idr_alloc2_test, 0, 1, GFP_KERNEL); + assert(id == -ENOSPC); + + id = idr_alloc(&idr, idr_alloc2_test, 0, 2, GFP_KERNEL); + assert(id == -ENOSPC); + + idr_destroy(&idr); +} and with this patch, it passes: +++ b/lib/idr.c @@ -40,6 +40,8 @@ int idr_alloc_u32(struct idr *idr, void *ptr, u32 *nextid, if (WARN_ON_ONCE(!(idr->idr_rt.xa_flags & ROOT_IS_IDR))) idr->idr_rt.xa_flags |= IDR_RT_MARKER; + if (max < base) + return -ENOSPC; id = (id < base) ? 0 : id - base; radix_tree_iter_init(&iter, id);