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bh=/NV95irI55Pn1dIUahTxJbOhFuCF2L936tCyrHrEDOw=; b=mWuKe3fMkNYKRWGZTn06GzCwN3 c5T6RoXqWCwAiaZgUGe+oeBPGkveBahHBRpq4j6S5lYN0oqQmJA+0AoxXbTNuts07VNQpkUSFtr4E MWyYN2kg+Ti/16X88hmrTqnH7y2YR8+f0VfHEn+IVtMpMizG0WMYOgP47U0w+MG5Z60BjCLzM60kW j9woTWwVisxlzOR/ppM2pzIU6TwR05hzpZLXmVL7YJBTZ/KvcrVXoOuKjbrUvYZcG8U/c/vErGKe3 ak83Ewy7gog65JpqXdbRueRrUy0dACvWCZp1N2jTxC7M5D7NMh/E0dbAm6TbHeSy/k0Owud3QFtKx tSxDN0OA==; Received: from willy by casper.infradead.org with local (Exim 4.98.2 #2 (Red Hat Linux)) id 1uJdMv-0000000BiV3-36jl; Mon, 26 May 2025 19:20:17 +0000 Date: Mon, 26 May 2025 20:20:17 +0100 From: Matthew Wilcox To: David Hildenbrand Cc: Dev Jain , ziy@nvidia.com, dhowells@redhat.com, hughd@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, ryan.roberts@arm.com, aneesh.kumar@kernel.org Subject: Re: [QUESTION] xas_reload() in iter_xarray_populate_pages() Message-ID: References: <20250526063524.22597-1-dev.jain@arm.com> <8da60934-0670-4f8a-8bde-fa4de320cdbb@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <8da60934-0670-4f8a-8bde-fa4de320cdbb@redhat.com> X-HE-Meta: U2FsdGVkX1+85NTi1o5BTE8iGN8fRLxJHTxOvwX8hhpRZaPc2B2vxqhmuoNOjD1a69rsrvPZzU4looSp6Gc0VSBeLxoENsI9AheJ7D7kaZwJFGSt+dkOVmGAmKy7v/uglVllrdrFP9JWAVjkd5dVZZtLsHGoV8XYN1rFrVnjm+A+4sy4tEMVcWjLQCgNXBY3O9/9T4a35dcOXr+qkTbg/5VbsnmpmTGBeEcU1A3lE0L8b23chXl9Vk9kLDBmDbQYZPrbXE9ODyoCdSTmZSCT2qd0JUpjx2Jiyb64IJ7FKl7ZoBpoL6pC4nIqdL9F23F2A7bJvGshDKZjUG1jd1r/R8e7y3awIJIkeuN3661aISr380LxvUVgjU1FPAeWcnazVOhPG4Q+/j+pzHtHnoFTo+aySPRAHTulO7Lde/7YbITsd6LZ3OymVagwpJ/fcnyU0gWn8xO4ACQmKL5xPznKvksePpLgKOJkXj6ZDSRH3FW8guku0WFTb+7m+kqzsPBdwYqTnWqKFDNzfN3re6j9p3WUMlfnEy2pf13jmTno3J+Js0qqGDQMz6C188L6EGBK9HcHhyEo17wCkIGZkxNKhXalpPvimJC+JOMIDGMOG0yfyCeNbTh3K0ejH26QohXJpyI4TvY9P/d01ldiRyQ+NL76s/9ZqdsaNVzFI3PwKC8sgcGYjDLgnVyvg8yrPamAq5dYnfebr/iSK4A1wykA7qdSwtniSh8HnhdsUL0lY/ZmHkoQ2kDfcxpJboqMqgXGPT1usSvOSiCbmBUPndnhG+xBJ8vFrE/PjY/AU+czyvcgSHtnOvGkezGl1HFSMiFSRtgkl3wpoKGF2T0LrbZn3wVU6asjsPrkiTdTt1O3cSxwN0pquOmzoawRrGcLO2De3AwOTHwEOGBteK7tZE2uuEYPoyd1RY9yW25mf1IbmF2QLQWdtk6IES1Fruz9C+eXMo9IPl/DMrPkitcSRwL 5U8yNhdS E/C2T9bllXJpP7o6yvL3M8K1tzyWpTC7k82XV7gZwNfVuP4BVDrjz8AeOA84h9R58Jafg1Rfo4D2kLkL+fNcMX+8BRuDgJh3uWDQDTrbrc+mHiVtydkmGwMprptGraHW6+CcXeZ0msMo92VM9n+9/eSPW3PFFXYrDH2CmcPTcrqJuDg8+HvqCyyMsGUjz+AHFNMBWvM5M8f0Z27nad+kKulRjfdawLXzesohBEzikyGDYG+8gbhFe9qIK5PcJU2Bnwodudg1c83oEMUbtHmZyn/7E+g== X-Rspam-User: X-Rspamd-Queue-Id: 97B9740007 X-HE-Tag-Orig: 1748287225-457941 X-Rspamd-Server: rspam09 X-Stat-Signature: fbs1iuuahu6b9izmtyqkba4zjgywsxks X-HE-Tag: 1748287229-41596 X-HE-Meta: U2FsdGVkX19LTYYMEKI8Ecar/fICnPN0yoqVHsxPRiILrq00qKtNPiqE9tLoqWSjXeqiU7yaTosEDFcKmvqUrb+EcqEXdTq0RkwZK31EFYvO09cfigQv1tNgRl/3bFTSLj3vcTwgQwDjLhxeevcjIpiOLYC1gmvJzTg1BaS4rzTl087GqikD1s2TgIDngmwL2lM3HoQiaFo2ApGoJ8G/XhzWdbINVe2eCqVpzf7uonLmpsZGwoT027GgD7lFOp2L49taJzlSp+a9D/5QH7pYpEkyC89vOeWXXQQucSjfi/AwrouoawODm2pYm8dQ+6TB2F6vIGeeThD5raL3/7rbFzVtayvRhdJ2JrS6gQ/XKuC4V1TLzNvR4MSBHJxDxu6+IoNsf7mfWFbGGWwFR3+SwZ5BaJc1Vtvar74owdCbICnDoQByGsM4Il6ynoYKkVXz/Szb75YakaBN3KS5E1VXtL3gy7Ty14nj76CSVfImcLGr8EbEMeHxMvLOaysTr0RXV9sREWz+UON5iITP5y4bMctXZNdRsTbZdO2Zy4aD3p97YXWpG96UJ7ON8NbdsvPzC4xBZ5ToiKEY4QevlnaGhKKARypOH0aAh2oLdhbyc1f6w1aRp2VmvBPz4fy4EZdSchMdxjJcWTYdqeVToBr5ZAaGj6ebStYS9vkWihrMErPrFJ2NLKSeT8ZLPhwhAse/oUTPDrsMull7PK7jsej2Lr+3gQz/Hf7GX2+KoCawBbPZv9UfPSNmHhYp1vPUBsGT+JcOzPj+8o7A3OA8SNkp7m2xqLY8BKEtD54Mbl5JvCSdDR28y2n2KVzN6V9Oaau7uJ6OqDTaNQ4iVU1gNl2ADmBvJ0YQrwq76Glbc5UeJLX5+oHiMI/43pwqMKhIHIctB6EdvTcBceWgmTMPYzy6P3Q+PL3jl7zMdF9vGILjKY/bsEDs3budA0GQVO/Gg58ilc26PV5heYn/3G0Lmuh HjdbcZ14 1WMJmj/JW4derapBcYpArXrDdoX20QDy1iMkcBCZS0jxcQAE0HuHtGdwXMKKoX8NuUD3NOuTaGGBBCZQPUhdrWNAnAfuFT/5t4yrSPuW+30rWKtwIe2B+d6E7SFpwxg9RrgzTT4Y9kQmFwBWkMcIxgAJVfUJ49+oLLGawKseAofTPqWsTiLx84elL/NuWO0iW9/nQlJwpGQZIPrQ= 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 Mon, May 26, 2025 at 09:40:06AM +0200, David Hildenbrand wrote: > On 26.05.25 08:35, Dev Jain wrote: > > Hello all, > > > > After doing an xas_load() and xas_retry(), we take neither a reference nor a lock > > on the folio, and we do an xas_reload(). Is this just to reduce the time window > > for a race? > > > > If the above is true, then, there is a negligible window between xas_load() and > > xas_reload(), because only xas_retry() exists between them, so why to even reload()? > > The usual sequence for the pagecache is (see filemap_get_entry()) > > 1) xas_load(): Load the entry > > 2) xas_retry(): Test if we have to retry immediately > > 3) folio || xa_is_value(folio): check if the entry stores a folio > > 4) folio_try_get(): try getting a folio reference, might get freed > concurrently, so a folio_get() is not safe > > 5) folio != xas_reload(&xas): recheck whether the entry was changed > concurrently > > iter_xarray_get_pages()->iter_xarray_populate_pages() works on whatever > xarray was provided to iov_iter_xarray(). > > erofs/netfs/orangefs seem to pass the pagecache ... so I would also assume > that we have to use the same sequence as above. > > Willy and me had a look ad that code in b57f4f4f186d ("iov_iter: convert > iter_xarray_populate_pages() to use folios"). > > But looking at it now, I think that code is incorrect. At least the > folio_get() and reload-before-folio-get is weird. Well, I just converted it; I didn't think hard about what it was doing was right. I think I may have mentioned to dhowells that I thought the xas_reload() was unnecessary as the folios are required to be referenced by the caller. So if you can look them up, they're guaranteed to not be replaced [1]. [1] the xas_retry() cannot be skipped as it guards against a rearrangement of the tree which can happen even with the folio pinned.