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Tue, 28 Jan 2025 11:38:48 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id hUH6IsjBmGeDIQAAD6G6ig (envelope-from ); Tue, 28 Jan 2025 11:38:48 +0000 Message-ID: <86bed9d6-299f-4e28-8cb3-c60a5df78493@suse.cz> Date: Tue, 28 Jan 2025 12:38:48 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/5] mm: simplify vma merge structure and expand comments Content-Language: en-US To: Lorenzo Stoakes , Andrew Morton Cc: "Liam R . Howlett" , Jann Horn , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: From: Vlastimil Babka Autocrypt: addr=vbabka@suse.cz; keydata= xsFNBFZdmxYBEADsw/SiUSjB0dM+vSh95UkgcHjzEVBlby/Fg+g42O7LAEkCYXi/vvq31JTB KxRWDHX0R2tgpFDXHnzZcQywawu8eSq0LxzxFNYMvtB7sV1pxYwej2qx9B75qW2plBs+7+YB 87tMFA+u+L4Z5xAzIimfLD5EKC56kJ1CsXlM8S/LHcmdD9Ctkn3trYDNnat0eoAcfPIP2OZ+ 9oe9IF/R28zmh0ifLXyJQQz5ofdj4bPf8ecEW0rhcqHfTD8k4yK0xxt3xW+6Exqp9n9bydiy tcSAw/TahjW6yrA+6JhSBv1v2tIm+itQc073zjSX8OFL51qQVzRFr7H2UQG33lw2QrvHRXqD Ot7ViKam7v0Ho9wEWiQOOZlHItOOXFphWb2yq3nzrKe45oWoSgkxKb97MVsQ+q2SYjJRBBH4 8qKhphADYxkIP6yut/eaj9ImvRUZZRi0DTc8xfnvHGTjKbJzC2xpFcY0DQbZzuwsIZ8OPJCc LM4S7mT25NE5kUTG/TKQCk922vRdGVMoLA7dIQrgXnRXtyT61sg8PG4wcfOnuWf8577aXP1x 6mzw3/jh3F+oSBHb/GcLC7mvWreJifUL2gEdssGfXhGWBo6zLS3qhgtwjay0Jl+kza1lo+Cv BB2T79D4WGdDuVa4eOrQ02TxqGN7G0Biz5ZLRSFzQSQwLn8fbwARAQABzSBWbGFzdGltaWwg QmFia2EgPHZiYWJrYUBzdXNlLmN6PsLBlAQTAQoAPgIbAwULCQgHAwUVCgkICwUWAgMBAAIe AQIXgBYhBKlA1DSZLC6OmRA9UCJPp+fMgqZkBQJkBREIBQkRadznAAoJECJPp+fMgqZkNxIQ ALZRqwdUGzqL2aeSavbum/VF/+td+nZfuH0xeWiO2w8mG0+nPd5j9ujYeHcUP1edE7uQrjOC Gs9sm8+W1xYnbClMJTsXiAV88D2btFUdU1mCXURAL9wWZ8Jsmz5ZH2V6AUszvNezsS/VIT87 AmTtj31TLDGwdxaZTSYLwAOOOtyqafOEq+gJB30RxTRE3h3G1zpO7OM9K6ysLdAlwAGYWgJJ V4JqGsQ/lyEtxxFpUCjb5Pztp7cQxhlkil0oBYHkudiG8j1U3DG8iC6rnB4yJaLphKx57NuQ PIY0Bccg+r9gIQ4XeSK2PQhdXdy3UWBr913ZQ9AI2usid3s5vabo4iBvpJNFLgUmxFnr73SJ KsRh/2OBsg1XXF/wRQGBO9vRuJUAbnaIVcmGOUogdBVS9Sun/Sy4GNA++KtFZK95U7J417/J Hub2xV6Ehc7UGW6fIvIQmzJ3zaTEfuriU1P8ayfddrAgZb25JnOW7L1zdYL8rXiezOyYZ8Fm ZyXjzWdO0RpxcUEp6GsJr11Bc4F3aae9OZtwtLL/jxc7y6pUugB00PodgnQ6CMcfR/HjXlae h2VS3zl9+tQWHu6s1R58t5BuMS2FNA58wU/IazImc/ZQA+slDBfhRDGYlExjg19UXWe/gMcl De3P1kxYPgZdGE2eZpRLIbt+rYnqQKy8UxlszsBNBFsZNTUBCACfQfpSsWJZyi+SHoRdVyX5 J6rI7okc4+b571a7RXD5UhS9dlVRVVAtrU9ANSLqPTQKGVxHrqD39XSw8hxK61pw8p90pg4G /N3iuWEvyt+t0SxDDkClnGsDyRhlUyEWYFEoBrrCizbmahOUwqkJbNMfzj5Y7n7OIJOxNRkB IBOjPdF26dMP69BwePQao1M8Acrrex9sAHYjQGyVmReRjVEtv9iG4DoTsnIR3amKVk6si4Ea X/mrapJqSCcBUVYUFH8M7bsm4CSxier5ofy8jTEa/CfvkqpKThTMCQPNZKY7hke5qEq1CBk2 wxhX48ZrJEFf1v3NuV3OimgsF2odzieNABEBAAHCwXwEGAEKACYCGwwWIQSpQNQ0mSwujpkQ PVAiT6fnzIKmZAUCZAUSmwUJDK5EZgAKCRAiT6fnzIKmZOJGEACOKABgo9wJXsbWhGWYO7mD 8R8mUyJHqbvaz+yTLnvRwfe/VwafFfDMx5GYVYzMY9TWpA8psFTKTUIIQmx2scYsRBUwm5VI EurRWKqENcDRjyo+ol59j0FViYysjQQeobXBDDE31t5SBg++veI6tXfpco/UiKEsDswL1WAr tEAZaruo7254TyH+gydURl2wJuzo/aZ7Y7PpqaODbYv727Dvm5eX64HCyyAH0s6sOCyGF5/p eIhrOn24oBf67KtdAN3H9JoFNUVTYJc1VJU3R1JtVdgwEdr+NEciEfYl0O19VpLE/PZxP4wX PWnhf5WjdoNI1Xec+RcJ5p/pSel0jnvBX8L2cmniYnmI883NhtGZsEWj++wyKiS4NranDFlA HdDM3b4lUth1pTtABKQ1YuTvehj7EfoWD3bv9kuGZGPrAeFNiHPdOT7DaXKeHpW9homgtBxj 8aX/UkSvEGJKUEbFL9cVa5tzyialGkSiZJNkWgeHe+jEcfRT6pJZOJidSCdzvJpbdJmm+eED w9XOLH1IIWh7RURU7G1iOfEfmImFeC3cbbS73LQEFGe1urxvIH5K/7vX+FkNcr9ujwWuPE9b 1C2o4i/yZPLXIVy387EjA6GZMqvQUFuSTs/GeBcv0NjIQi8867H3uLjz+mQy63fAitsDwLmR EP+ylKVEKb0Q2A== In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: wedzopoznjuzpg4fa3es7zd9e57zntgn X-Rspamd-Queue-Id: 75ABE20008 X-Rspam-User: X-Rspamd-Server: rspam06 X-HE-Tag: 1738064330-687668 X-HE-Meta: U2FsdGVkX1+frvhhAS/edtBENPiOgtb0Ck+GSez7IAaCuvacQQHcIFJ3b+2O4M2ddcVB2XKcbmW77DzCHySrcyF+MnPh0CwZEdfHIhsCR0ycpWZB1746P75FF7GsjtIeEM4/RwO8GoNxwVa/YFYRwDIf8zOKqY9JXUQjaNlbdrqFYPTHwJmOkvTrLBDteRB4gKEiKX5IfqZ0uUB3itcSqrI5Dau37IdxS+E3Ru7l+xXl2gykvOdUG7XyfRm1J4F+SlTpNllbPllBJqcCjdMQvuQACStybQ0RW2QsH0qd6pxBm9kq6sxlLmkqOuNTDlaWASrJmEFjRpzu85QoxPucNo+TC1dSbd1nqCgYeIBDNM42GXo0bhO7wE339pjJzWI84wHz2rrfPS47cIc9n4Mjtbgx/HkKi7peRO81wTCviU0x84qod+WpvWOANnb+W1jM2KPkD28E+0FvAVTpADDMcdnESL1kJ99d+PAZUs0HSGw1reemlQqpzkqq0fkx2ifS0Y3g5KdNFe1NU8L2zTbTCpZH7x1aN6hgqIJeFTcKSKPLGxRExKzu5Q7ZiI+QEUHKI0cykyz+KWOLAxUeGQq6jFQZA9P3bMyjjRHcLKSnpXRcLF6stALhNub+nIPItBjNVMjhe+VSxpLFbY8Chv1BkH65xx+T8Nr/Bqdu+zKIQbpj9u8bmKKIeoVJpG3mE31BDq0v+FnQT5UUWP5uuYQjO4ZRf7Jf1BflXttKemKkyEoXouE5tq70T8bLnZ6OE48ZD2LY0AUHlIBlX+Pu3iKjbGCxhhQpt6RE8TVwiGYQ7rK4yAxO+U/WB+agwqOMaPtDifkpsI14HnuxrcHRafXiinWgLVxKMTRELHhKzxG5sXnCcKHxwuNNGZupE8PcQdD6k441Fscptz7D6wgO3RFUvSfJimEBW2bzyf6wNoA6tT5qhpQETfqPcfRLqkIScAN7DS2q22APB4e3MSi0D9p uvze7ECt 7LxM9f3KHzKuZKqm+j0fjwsD8CmxaiCxxe29HT/v/TwR5qVGZMh1I/p4Ud+TiXGhMJx2lq4Fyzj9K7MPAdKs9Fdewc49cCWKnwrkP+MacaRUWYEz4ag2YNVvaQKblvizVASXNKrSsJrKDEO8Osb3VZuARLaj9eKfNKqopME/2q1B/IHSmJ8slJX+Jpqk/AntR/G9BGelefE97pCXQ7Mq8D7yXM2I0fjMq+mB2xaUJcQ8LVwV4+b0QoukwDiI/MHC0fzS7P6H25gQWXFyNm1LbqGEJgOPYUUwHxigA/b3uy2gCq0JuaQWiryTAEw== X-Bogosity: Ham, tests=bogofilter, spamicity=0.002043, 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 1/27/25 16:50, Lorenzo Stoakes wrote: > The merge code, while much improved, still has a number of points of > confusion. As part of a broader series cleaning this up to make this more > maintainable, we start by addressing some confusion around vma_merge_struct > fields. > > So far, the caller either provides no vmg->vma (a new VMA) or supplies the > existing VMA which is being altered, setting vmg->start,end,pgoff to the > proposed VMA dimensions. > > vmg->vma is then updated, as are vmg->start,end,pgoff as the merge process > proceeds and the appropriate merge strategy is determined. > > This is rather confusing, as vmg->vma starts off as the 'middle' VMA > between vmg->prev,next, but becomes the 'target' VMA, except in one > specific edge case (merge next, shrink middle). > > Int his patch we introduce vmg->middle to describe the VMA that is between > vmg->prev and vmg->next, and does NOT change during the merge operation. > > We replace vmg->vma with vmg->target, and use this only during the merge > operation itself. Yeah that's much better. > Aside from the merge right, shrink middle case, this becomes the VMA that > forms the basis of the VMA that is returned. This edge case can be > addressed in a future commit. > > We also add a number of comments to explain what is going on. > > Finally, we adjust the ASCII diagrams showing each merge case in > vma_merge_existing_range() to be clearer - the arrow range previously > showed the vmg->start, end spanned area, but it is clearer to change this > to show the final merged VMA. > > This patch has no change in functional behaviour. > > Signed-off-by: Lorenzo Stoakes Reviewed-by: Vlastimil Babka > --- a/mm/vma.h > +++ b/mm/vma.h > @@ -69,16 +69,48 @@ enum vma_merge_flags { > VMG_FLAG_JUST_EXPAND = 1 << 0, > }; > > -/* Represents a VMA merge operation. */ > +/* > + * Describes a VMA merge operation and is threaded throughout it. > + * > + * Any of the fields may be mutated by the merge operation, so no guarantees are > + * made to the contents of this structure after a merge operation has completed. > + */ Well this patch seems like a step in the direction to limit what's mutated, and perhaps defining some of the guarantees (via const?) could be then possible? > struct vma_merge_struct { > struct mm_struct *mm; > struct vma_iterator *vmi; > - pgoff_t pgoff; > + /* > + * Adjacent VMAs, any of which may be NULL if not present: > + * > + * |------|--------|------| > + * | prev | middle | next | > + * |------|--------|------| > + * > + * middle may not yet exist in the case of a proposed new VMA being > + * merged, or it may be an existing VMA. > + * > + * next may be assigned by the caller. Caller of what?